Heat the butter in a heavy-bottomed medium saucepan over medium heat. Add the onion and cook until golden – about 8 minutes. Stir in the garlic and chopped tomatoes and cook for another 2 minutes.
Add the chili sauce, vinegar, molasses, mustard, and Worcestershire sauce to the pan. Bring to a simmer and reduce heat to a medium-low. Stir frequently and continue to simmer until the sauce is slightly reduced – about 20 minutes.
