Preheat oven to 425. Add tomatoes, onions, and garlic to a glass 13X9 baking dish. Drizzle with olive oil, vinegar, and salt. Bake at 425 for 30 minutes.
Add contents of baking dish, basil, and tomato paste to a deep bowl. Puree using an immersion blender.
To freeze
Add pureed tomato mixture to freezer bags. Freeze until ready to use.