Roughly chop tomatoes. Skin and halve onion. In a medium saucepan, combine tomatoes, onion halves, and butter over medium heat. Once the tomatoes start to boil, lower heat to a gentle simmer. Simmer, stirring every 10 minutes or so, until the tomatoes are completely broken down and the sauce is thickened, about 40-50 minutes. Remove from heat. Season with salt to taste.
Notes
Source: adapted from Marcella Hazan
Recipe by Wright Family Table at https://www.wrightfamilytable.com/heirloom-tomato-butter-and-onion-sauce/