Preheat oven to 350 degrees F. Coat a 2-quart casserole dish with non-stick cooking spray.
Heat a large skillet over medium heat. Place 8 bacon slice halves in the skillet and cook until crispy, turning once, about 7 minutes. Remove to a paper towel lined plate to drain. Once cool, crumble the bacon.
Reserve 1 tablespoon bacon fat, and discard the rest. Add onion to the hot bacon fat in the skillet, and cook for 5 minutes, or until onions become soft and translucent. Add ground beef to onions, break apart ground beef and cook until beef is no longer pink, about 8 minutes.
In a large mixing bowl, combine pork and beans, tomatoes, brown sugar, mustard, salt, and pepper. Add crumbled bacon and beef mixture to the large bowl. Stir to combine.
Pour bacon, beef, and bean mixture into the prepared casserole dish. Lay remaining 4 bacon strip halves on top of the beef mixture.
Cover the dish with tin foil, and bake for 45 minutes. After baking has finished, carefully remove foil, and set oven to broil. Cook until bacon on top is crispy, about 3 to 5 mins.
Serve hot over mashed potatoes or spoon onto burger buns and top with a slice of cheese.