Ingredients

1 tablespoon extra-virgin olive oil 

1 medium yellow onion, grated 

2 1/2 pounds fresh or frozen heirloom tomatoes, grated 

2 tablespoons unsalted butter 

Coarse salt and freshly ground black pepper 

Crushed red pepper flakes

Preparation

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.