Saute minced onion -- I use a full Mayan sweet and diced green bell pepper in a bit of olive oil with minced garlic -- I use about 2 heaping teaspoons of the refrigerated jar kind. Add sliced fresh mushrooms or a can if that's what you have. Add 2 cans 14oz tomato sauce and 1 can of diced tomatoes---sometimes I do 1 sauce, 2 diced---and 1 can tomato paste. Season with about a tablespoon or so (I don't measure I just add a mound from the palm of my hand) italian seasoning, a few shakes of oregano and basil and a teaspoon or so of sugar. Simmer, taste and add what you need according to taste.
If we don't have meatballs I use ground Italian Sausage, add it after the veggies and brown it before adding the sauce.