Melt the butter with the olive oil in a non-reactive (enamel, glass or stainless steel) pot on medium heat. Add the onion and garlic and cook, stirring occasionally, for 5 minutes.
Cut each tomato into 16 or so chunks and add to the pot. Bring the sauce to a boil, then turn down the heat until it is at a very low boil. Add the bouquet garni. You can also add a small amount of sugar if the tomatoes are not sweet enough.
Cook the sauce at this very low boil for 1/2 hour, stirring occasionally. Remove from the heat and allow to cool to room temperature.
Using the finest mesh strainer on a food mill, process the sauce into a bowl. Season to taste with salt and pepper.
You can freeze this sauce in clean plastic containers.
Makes about 2 cups.
(If you don't have a food mill, be sure to peel and seed the tomatoes before adding them. After it is cooked, run the sauce through a food processor to smooth it.)