Mix together ground beef, egg yolk and salt and pepper.

Form ground beef mixture into a patty, making sure the center of the patty is less thick than the edges as the center of the patty will swell to a greater degree than the outer area when cooking.

Cook burger on the grill, in a grill pan or skillet to required doneness. Use just a touch of cooking oil and allow to rest.

While patty is resting, mix together tomatoes, scallion, ketchup and mustard and set aside.

Cut the potato bun in half and toast the insides using whichever method you used to cook the burger.

To stack the burger, place lettuce on top of base of bun, overlap lettuce as necessary. Place a slice of soppressata on top of lettuce, then place patty on top and then add another layer of soppressatta. Carefully spoon tomatoes on top and then press the top of bun to crown the burger. Use a skewer to keep in place if needed.