Heat 3 tablespoons butter and 3 tablespoons olive oil in a large saucepan until the butter has melted. Adjust the heat. You don’t want the butter to brown, just a light sizzle.
Add about 1-½ cups diced shallots (or onion) and cook for 3 minutes until they become soft.
Stir occasionally and season with ¼ teaspoon kosher salt and ¼ teaspoon freshly cracked black pepper.
Add 1 tablespoon minced garlic (about 2 cloves), 1 tablespoon dried oregano, and ½ teaspoon red pepper flakes.
Cook and stir for one minute.
Add a 28 ounce can of peeled whole plum tomatoes and a 15 ounce can of tomato sauce.
Bring to a boil, give a good stir, reduce heat, and let simmer for 30 minutes. While it’s simmering, occasionally break up the tomatoes using a wooden spoon or tongs.
After 30 minutes, taste, season to your liking, and taste again.