01 - In a large skillet, cook chopped bacon over medium heat until crisp. Add ground beef and continue cooking, breaking up the meat, until browned. Drain excess fat.
02 - Add chopped onion to the skillet with the cooked meat and sauté until translucent, about 5 minutes.
03 - In a large mixing bowl, combine baked beans, kidney beans, butter beans, ketchup, brown sugar, vinegar, mustard, salt, and black pepper. Mix thoroughly.
04 - Stir the cooked meat and sautéed onions into the bean mixture until evenly distributed.
05 - Transfer the mixture to a 2-liter (2-quart) baking dish. Bake in a preheated oven at 175°C (350°F) for 45 minutes, uncovered, until bubbly and heated through.
06 - Allow casserole to rest for 10 minutes before serving to enhance flavor development.