Simple tomato sauce

From Anne's Food
2 cans (each 400 g) whole tomatoes 70 g butter 1 yellow onion salt Peel the onion and cut in half. Place in a pot with the tomatoes and the butter. Bring everything to a boil, lower the heat and let it simmer on medium heat for 40-45 minutes. Give it a stir every once in a while, and crush the tomatoes with a spoon. When you see little droplets of fat on the surface, you're done. Serve with freshly boiled pasta. And if you want, add parmesan and black pepper, but frankly, I liked this better on its own.