Step 1

Pulse tomatoes in a food processor until a smooth purée forms. Heat oil and butter in a medium saucepan over medium-high. Add onion and cook, stirring, until golden and soft, 5–7 minutes. Add garlic and cook, stirring, until softened but not brown, 1–2 minutes. Add tomato paste, sugar, and red pepper flakes; cook, stirring, until sugar dissolves, about 2 minutes. Add tomato purée and season with salt. Reduce heat and simmer, stirring occasionally, until sauce has thickened, 15–20 minutes.

Step 2

Do Ahead: Sauce can be made 3 days ahead. Store in an airtight container and chill, or freeze up to 2 months.