Preheat oven to 350 degrees F (175 degrees C). Spray a casserole dish with cooking spray.
Cook and stir ground beef and onion in a skillet over medium heat until beef is browned and onions are translucent, 10 to 15 minutes. Drain well.
Mix pork and beans, kidney beans, ketchup, brown sugar, mustard, vinegar, and salt in a large bowl. Stir hamburger mixture into bean mixture. Pour hamburger-bean mixture into prepared casserole dish.
Bake in the preheated oven until bubbling, about 45 minutes.
