Preheat oven to 350°. Grease baking dish with butter.
In a deep skillet over medium heat, cook ground beef until no longer pink, breaking it apart as it cooks.
Add onion and green bell pepper to the skillet. Continue cooking until vegetables are softened. Drain and discard excess fat.
Stir in ketchup until well combined. Add mustard, Worcestershire sauce, and brown sugar. Mix thoroughly. Season with salt and freshly ground black pepper to taste.
Add undrained pork and beans to the skillet. Stir gently to incorporate without breaking beans.
Transfer the mixture to the prepared baking dish. Spread evenly and cover tightly with aluminum foil. Bake for 45 minutes in the preheated oven.