Preheat the oven to 350 degrees F (175 degrees C). Cook bacon in a large skillet or in the microwave until much of the grease has been released, but the bacon is still flexible. Drain on paper towels and set aside.
In a 9 inch square baking dish, stir together the baked beans, onion, mustard, ketchup, brown sugar and apple. Top with slices of bacon.
Bake uncovered for 45 minutes in the preheated oven, until the bacon is crisp and beans are bubbling hot.