Step 1

If using fresh tomatoes: Using a sharp knife, cut an 1/4-inch deep X on one end of each tomato. Then, place scored tomatoes into a pot of boiling water and cook until you see the skin starting to wrinkle and split, 1-3 minutes, then lift the tomatoes out with the slotted spoon and plunge them into a bowl of ice cold water. Peel the skin off of the tomatoes and cut into 1/2-inch chunks. (Cut off and discard the cores at this point.)

Step 2

Place the diced tomatoes and all of their juices into a large saucepan. Add the butter, olive oil, onions, garlic, salt and sugar and bring to a gentle boil. Reduce the heat to low and simmer, uncovered, for 75-90 minutes, stirring occasionally, or until the sauce is no longer watery. Remove and discard the onions and any large chunks of garlic (the garlic should have mostly dissolved into the sauce). Taste and adjust seasoning if necessary. Add basil right before serving.