This basic tomato sauce recipe is as easy as it gets, and it's a wonderful way to use frozen tomatoes.
Heat olive oil in a medium saucepan over medium heat. Add onion and cook until translucent. Add tomatoes and butter; cook, stirring, until butter has melted. Season with salt, pepper, and crushed red-pepper flakes. Let cook until sauce has thickened, 25 to 30 minutes.