In a small saucepan over medium heat, melt the butter. Add the onions and garlic and cook, stirring, until softened but not colored, about 3 minutes. Stir in the ketchup, sugar, lemon juice, Worcestershire, and mustard and bring to a boil. Reduce the heat to low and simmer, uncovered, until thickened, about 15 to 20 minutes.