In a skillet, saute the onion until it becomes transparent, and then add the ground beef. Cook until all the ground beef is browned. Making sure to break up all the large pieces. Drain off all the fat and set aside. Open all the buns and lay them insides facing up. Now on the tops you want to spread about 1 tsp of mustard. (More or less depending on your likes.) Then place 3 sliced pickles on top. Now you will need to place one unwrapped slice of cheese on each bun top. Take the bottom pieces now and scoop on the ground beef mixture on. Place the tops on and then wrap in the wax paper. (For best results bring up both sides to meet in the middle then roll those together until flat on top of bun and then fold the sides under.) Bake on a cookie sheet at 350 until cheese is melted and bun is toasted. Enjoy with french fries, macaroni salad or even potato salad.