Preheat oven to 425˚ F. Toss tomatoes with chopped onion, chopped garlic, olive oil, and salt. Place in oven and bake for 30-35 minutes until tomatoes are tender and starting to pop. Remove from oven and let cool for 5 minutes. Puree in a blender with your choice of mix-ins. Store for up to a week in the refrigerator or up to six months in the freezer.