Potatoes au Gratin with Caramelized Onions and Roasted Garlic

Name: Potatoes au Gratin with Caramelized Onions and Roasted Garlic
Prep time: 30 min
Cook time: 135 min
Total time: 180 min
Ingredients: 4 to 5 heads of garlic; 4 to 5 tsp olive oil; 1 pinch salt; 5 small sweet onions (sliced); 1 tbsp butter (unsalted); 1 tbsp olive oil; pinch salt (optional); 3 tbsp butter unsalted; ¼ cup all-purpose flour; 1 cup chicken broth (low sodium); 2 cups milk; 1 tsp salt; ¼ tsp pepper; 5 sprigs fresh thyme; 5 sprigs fresh rosemary; 3 lbs Yukon gold (sliced into ⅛ inch rounds); 1½ cup sharp cheddar cheese (freshly shredded); 1 cup mozzarella (freshly shredded); cooking spray

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If you’re looking for a delicious and flavourful side dish then this Potatoes au Gratin with Caramelized Onions and Roasted Garlic is it! Perfect for holidays or anytime you need a delicious side!

Is there anything better than a potato-based side dish? Usually for the holidays, I keep it simple with mashed potatoes or baked potatoes but this time around, I felt like changing things up with this potatoes au gratin.

Why You’ll Love This Side Dish

It’s a satisfying dish. I won’t lie to you, this is a heavy side since there’s potatoes, cheese, and a béchamel sauce but boy is it delicious.
It’s not the prettiest side dish but it’s a super tasty! I won’t say it’s an easy recipe per say as it’s a little time intensive, but it’s tasty enough that it’s worth the effort. This dish is adapted from Craving Home Cooked. It’s already a tasty dish to start but I add caramelized onions, roasted garlic, and infuse herb flavours into the béchamel sauce which require an extra hour of work. Luckily, you can caramelize the onions while the garlic is in the oven.
Components can be made ahead of time. Yes, I just said it takes an extra hour than your traditional potatoes au gratin but caramelized onions and roasted garlic can be made ahead of time so you break up the overall prep work.

Ingredients You’ll Need

Ingredients needed to make potatoes au gratin.

caramelized onion — to make caramelized onions, you’ll need 5 small onions, butter, and olive oil. You can see step by step photos in my how to make caramelized onion post.
roasted garlic — to make roasted garlic, you’ll need two heads of garlic as well as olive oil and a pinch of salt. You can see step by step photos in my how to roast garlic post. You can also roast garlic in an air fryer.
butter — unsalted butter is ideal as we are already adding salt to the béchamel sauce.
all-purpose flour — you’ll need flour to make the roux for the béchamel sauce.
chicken broth — I use low sodium chicken broth whenever possible.
milk — I use whole milk.
fresh herbs — I use rosemary and thyme.
salt and pepper — you can swap the pepper for white pepper if you want it to blend into the béchamel sauce.
Yukon gold — you can use Russet potatoes as well.
cheese — I use both cheddar and mozzarella sauce.
fresh thyme for garnish if desired
cooking spray

How to Make Potatoes au Gratin with Caramelized Onions and Roasted Garlic

Caramelized onion on a plate with a spoon.

Make the Roasted Garlic

Heat the oven to 400F.
Remove the paper-thin skin of the garlic. Trim the top ¼ inch off the top of the garlic bulb.
Place the heads of garlic onto a piece of tin foil and drizzle with olive oil, and sprinkle with salt. Tightly wrap the garlic in the foil.
Roast in the oven for 40 to 60 minutes. When cool to touch, remove the garlic cloves from the skin and set aside. You can squeeze the cloves out or simply peel the skin off. Set aside for later.

Make the Caramelized Onions

Add the onions and sauté for 5 minutes or until the onions have softened.
Add the salt, if using, and reduce the heat to medium low and cook for 50 to 60 minutes. Stir the onions with a wooden spoon or spatula every few minutes, or until the onions are golden brown, and caramelized.
Set aside for later. Before adding to the baking dish, pat the onions with some paper towels to absorb any extra moisture.

Make the Potatoes au Gratin

White sauce with herbs infused in a pot.

In a small pot on medium low heat, melt the butter before adding the flour, bit by bit, and stir until evenly combined. Cook for a minute.
Slowly pour in the chicken broth and whisk until combined.
Slowly add in the milk, and whisk until combined.
Add the salt, pepper, and fresh herbs and allow fresh herbs to infuse for 2-3 minutes on low heat, stirring constantly. Do not let it come to a boil. Remove from heat and remove fresh herbs before setting aside.
Spray a 9×9 or 9×13 baking dish with non-stick spray.

White sauce added to the potatoes with caramelized onions and roasted garlic.

Spread half of the sliced potatoes in an even layer on the bottom of the pan and top evenly with the caramelized onions, roasted garlic, and half of the béchamel sauce.
Top with half of the cheese.

More potatoes layered into the baking dish.

Layer the rest of the sliced potatoes on top before adding the rest of the béchamel sauce and cheese.
Cover with tin foil and bake for bake for 45 minutes. Remove the tinfoil and bake for another 30 minutes, or until the potatoes are cooked through.
Let rest for 15 minutes before slicing and serving. Top with some fresh thyme if desired.

Recipe Tips and Notes

I use a 9×9-inch baking dish but it goes right up to the top. If you don’t want to risk overflow, use a 9×13-inch baking dish.
I use a mandolin slicer to slice my potatoes evenly.
I always buy extra potatoes as I don’t run the entire potato through the mandolin slicers as I prefer using the larger slices of the middle portion of the potatoes (and protecting my fingers lol).
Resting before slicing gives the cheese time to set so the potatoes don’t fall apart as you serve them.
You can mash the roasted garlic into a paste and mix it into the béchamel sauce. I just like having little surprise bits of cloves when I take a bite out of the potatoes au gratin.
I highly recommend grating your own cheese instead of using a bag of pre-shredded cheese. The cheese melts much better as there isn’t an anti-clumping coating on cheese you shred from a block yourself. It’s super easy if you have a food processor as you can use the shredder blade and just run the block of cheese through it.

A plate of potatoes au gratin with caramelized onions and roasted garlic.

Make Ahead Tips

Make ahead: make the roasted garlic and caramelized onion up to 3 days ahead of time. Store it in an airtight container in the fridge.
Freezer: the potatoes’ texture may not be the same after freezing. If you’re ok with that, this potatoes au gratin can be frozen for up to 1 month after its been baked.

What does potatoes au gratin mean?

Potatoes au gratin or au gratin potatoes means “covered with bread crumbs or grated cheese and browned” according to Merriam-Webster.

Do I peel the potatoes?

For thin-skinned potatoes like Yukon Gold, I don’t peel the potatoes. Simply give them a good scrub and pat them dry before slicing. If you use russet potatoes, peel them before slicing as the skin is much thicker.

Cheeses to use for potatoes au gratin?

You can use any type of melty cheese such as gruyère, parmesan, smoky gouda, or spicy cheese for a kick of heat.

What is the difference between scalloped and au gratin potatoes?

According to Southern Living, potatoes au gratin are more decadent as they are made with lots of cheese in between the layers of potatoes and on top as well.

Serving Ideas

Here are a few dishes that go well with potatoes au gratin:

Turkey: try my Instant Pot turkey breasts and gravy or oven roasted turkey thighs.
Chicken: try my Dutch oven chicken, roasted chicken quarters, or crispy chicken drumsticks.
Sides: as these potatoes are heavy, I’d go with lighter side dishes such as maple Brussels sprouts, this silken tofu recipe, oven roasted beets, seasoned spinach, or asparagus.