In a skillet, cook ground beef and chopped onion over medium heat until beef is browned and onion is tender. Drain excess fat.
In a large bowl, combine cooked beef mixture, kidney beans, pinto beans, butter beans, ketchup, brown sugar, Worcestershire sauce, salt, and black pepper. Mix well.
Transfer the mixture to a baking dish. Bake in preheated oven for 45 minutes, until bubbly and heated through.
Let cool slightly before serving. Enjoy!
Notes:
This casserole can be made ahead and refrigerated. Just bake it when you're ready to serve.