Heat tallow in a sauce pan over medium heat.

Sauté garlic for 30 seconds, then toss in onions. Stir ever once in a while, cook for 10-15 minutes till onions start to brown and caramelize.

Add tomatoes and basil, simmer for 45 minutes or so.

If you want the sauce thin, run an immersion blender through it (or put it in a blender.)