serves 6
2 pounds ripe tomatoes, blanched, peeled, cored, and roughly chopped (about 4 1/2 cups), or 1 (28-ounce) can imported Italian plum tomatoes (such as San Marzano) (undrained), chopped
1 medium-size (10-ounce) yellow onion, peeled and cut in half lengthwise
5 tablespoons unsalted butter (2 1/2 ounces)
1 teaspoon kosher salt, plus more to taste
Place tomatoes in a medium saucepan; add onion, and stir in butter and salt. Cook over medium-low, uncovered, stirring occasionally and mashing any large pieces of tomato using the back of a wooden spoon, until fat floats free from the tomatoes and is incorporated into the sauce as it slightly thickens, 1 hour to 1 hour and 10 minutes. Season with additional salt to taste. Remove and discard onion.
To make ahead
Finished sauce can be frozen in an airtight, freezer-safe container for up to 1 month. Be sure to remove and discard the onion before freezing.