Tomato Sauce with Butter and Onion

To prepare the tomatoes, if using fresh

1. Wash tomatoes. Insert the tip of a sharp paring knife into the stem end of each tomato, at a 45-degree angle, and carefully cut around the stem. Discard cores. (This step is optional, but is nice to do to avoid tough core pieces in the sauce. If you don't core the tomatoes, gently score them by cutting a shallow "X" into one end.)
2. Bring a large pot of water to a boil. Add tomatoes, and blanch until skins begin to blister and split (anywhere from 30 seconds to 2 minutes). Remove tomatoes with a slotted spoon to a bowl of ice water. Let cool briefly and slip the skins off of each tomato (they should slide right off; any stubborn spots can be helped along with a paring knife.) Place in a large bowl or dish, allowing juices to accumulate.

To make the sauce

1. Place tomatoes and their juices (fresh or canned), butter, and onion, in a heavy-bottomed saucepan. Bring to a simmer over medium heat, lowering temperature as needed to maintain a gentle and constant bubble. Simmer, uncovered, for about 45-minutes, until sauce is reduced and small pools of butter rise to the surface. Stir the sauce every 10-15 minutes as it simmers, breaking up tomato pieces against the side of the pot with a wooden spoon.* Do not allow the sauce to scorch.
2. Remove onions and eat or discard. Season the sauce with salt, to taste, and serve.