Combine olive oil, red pepper flakes, minced garlic, onion, butter, italian seasoning, oregano, sugar, salt, and pepper in a large pot over low heat. Cook for about 5 minutes, stirring frequently. (you want them to lightly brown but not too dark)
Add crushed tomatoes and increase heat to a medium-low.
Simmer for an hour and half to two hours (longer the better)- then stir in Parmesan cheese.