1. Heat the olive oil and butter in a large pot.
2. Stir in the chopped onions and garlic and sweat them over medium heat with the lid on.
3. When the onions are tender, add the chopped canned tomatoes. (A hand blender works well for chopping.)
4. Add sugar and salt and pepper to taste.
5. Simmer covered for 1/2 hour. If you will be using it in a recipe that calls for further cooking, you can reduce simmering time to 15 minutes.

John Bishop writes, "This sauce can be made ahead and kept refrigerated for 5 days. I make it every week because it's so handy to have around...The secret is to keep the lid on while it's simmering. It results in a bright red, intensely flavoured sauce."