2 tablespoons butter
1/2 small onion, chopped
1/2 green bell pepper, chopped
3/4 cup ketchup
1 tablespoon cider vinegar
Melt butter in a large skillet over medium-high heat; add onion and bell pepper, and sauté 5 minutes or until vegetables are tender. Stir in ketchup and vinegar, and simmer, stirring constantly, until sauce thickens.
For a spicier sauce, substitute 3/4 cup chili sauce for ketchup.