Preheat oven to 400 °F (200 °C).
In a rimmed baking pan, mix together tomatoes and garlic. Melt 2 tablespoons (30 g) of the butter and pour it on top.
Mix well then roast the tomatoes for forty-five minutes, stirring occasionally, until the tomatoes are soft.
Let the tomatoes cool.
Scrape the tomato and garlic mixture into a food processor with the salt. Pulse until a smooth puree, about twenty-five seconds.
Add butter in small chunks and process until completely mixed in with the tomato.