Preheat the oven to 350 degrees. Heat a large skillet to medium-high heat. Add the bacon pieces and fry until crispy. Drain on paper towels. Add the onions to the same pan with the bacon grease and sauté onion until brown.
Mix the bacon, onions, and bacon drippings with the pork and beans, mustard, maple syrup, brown sugar, ketchup, and lemon juice. Pour into a greased casserole dish and cover. Bake covered at 350 degrees for 45 to 60 minutes.