In a mixing bowl, combine all the ingredients for the burgers and mix thoroughly. If necessary, add a bit more oats until you achieve a fluffy dough that can be shaped. Be sure to grate the apple at the last minute to prevent it from turning brown. Cover the meat mixture and refrigerate it for 1 hour.
Divide the mixture into approximately 75g portions and shape them into 20-22 burgers. Place them on two large pans and bake in a preheated oven at 180°C (350°F) for 20 minutes.
Remove the burgers from the oven and place a slice of cheddar cheese on top of each one. Use a knife to create scary Halloween faces on the cheese slices. Return the burgers to the oven and continue baking for another 5 minutes, or until the cheese melts.
Remove from the oven. Slice the burger buns in half and spread mayonnaise on one half and ketchup on the other half. Place a cheeseburger on top and serve.
Enjoy your spooky Halloween cheeseburgers!