Start by cleaning, debearding, and soaking your mussels.
In a dutch oven (or large pot), add the oil and onion. Let the onions sweat for a minute.
Add in the chili flakes and garlic to the pot and cook for around 2 minutes.
Add in the can of diced tomatoes and water. Stir and bring the mixture up to a simmer.
Once it’s reached a simmer, add in the mussels and cover the pot. Allow the mussels to steam inside the covered pot for 7-8 minutes, mixing halfway through. Top with some parsley and more chilling flakes if you desire, serve immediately.