Gather all ingredients.

Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x5-inch loaf pan with cooking spray.

Mix beef, onion, stuffing mix, and egg by hand in a bowl. Press beef mixture into the prepared pan.

Bake in the preheated oven until no longer pink in the center, 35 to 40 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).

Top meatloaf with shredded mozzarella cheese and let rest while cheese melts, about 10 minutes.