Heat the butter in a saucepan. As soon as everything has melted, add the chopped onion and the chopped garlic and sweat until translucent. Then stir in the flour and make sure that nothing burns. Strain the tomatoes as desired and add to the roux with the tomato paste. Finally pour in the vegetable stock and season the sauce with just salt and pepper. Let everything boil again until a nice bond is formed.
Macaroni, which you toss in butter after cooking, go best with this.