For the stuffing
In a large bowl, combine the ground beef, the onion, the garlic, 1/2 cup breadcrumbs, the parsley, the egg, the fennel seed, gruyere cheese, salt, pepper. Mix until well combined.
Shape the meatballs mixture into balls, and then press a dice of cheddar cheese into each.
Once all of the meatballs have been stuffed, roll them in the flour, then in the yolk and finely in the breadcrumbs.
Fry in hot olive oil until golden brown on all sides and drain on a paper towel. Serve piping hot