Tomato sauce with butter and onion

Original recipe from smittenkitchen

Tomato sauce with butter and onions:
1 (28 oz) can diced tomatoes
1 onion, peeled and halfed
5 oz butter
3 cloves garlic

Combine tomatoes, onion, butter, and garlic in a pot. Bring to a simmer and simmer on low for about 45 minutes, until it's done.