Add olive oil to a saucepan (at least 3 qt) over medium heat.
Add the diced onions and saute until softened, for around 5 minutes. Add a pinch of salt.
Once soften, add the garlic and cook until fragrant, around 1 minute.
Add the canned plum tomatoes, tomato paste, and basil, and bring the pot to a boil before lowering the heat to simmer the sauce.
Stir occasionally, breaking up the tomatoes with a wooden spoon by pressing them into the side of the pot. Be sure to give the sauce a taste halfway to see if it needs any additional salt.
After around 20 minutes, your sauce should have thickened. If you want a smoother sauce, you can use an immersion blender on the sauce.