Yes, that's it. Just tomatoes, butter and onion. Pour a can of whole peeled tomatoes, with their juice, into a pan. Add 5 tablespoons of butter and half an onion (peeled, but not cut). Simmer, uncovered, for 45 minutes until thickened. Stir occasionally to break up tomatoes.
Discard the onion (save it for later!), and serve over cooked pasta. This serves 3-4 people, depending on how much sauce you like on your pasta. Even your pickiest of eaters will like it!