In a bowl, combine egg yolk, ground beef, onion, salt and pepper. Shape mixture by teaspoonfuls into 24 balls. Remove crusts from bread; roll flat and cut into 1-1/2-in. rounds. Place meatballs on bread rounds; make a depression in each ball and fill with a cube of cheese, making sure bread is covered with meat mixture.
Place on a baking sheet. Broil in preheated oven about 6 in. from heat for 3-5 minutes or until no pink remains. Garnish with ketchup, mustard, sliced green onions or sliced dill pickles if desired.