Preheat oven to 350°F and evenly mix the ground beef, bread crumbs, onions, eggs, milk, parsley, mustard, and spices together in a large bowl.
Line a sheet pan with parchment paper and use half of the ground beef mixture to create two 'soles.'
Cut the leeks into 3 to 4-inch pieces, remove the inner layers, and stuff them with the mozzarella. You'll need 8 stuffed leek rolls.
Place 3 stuffed leek rolls in the middle of each ground beef sole, cover them with the remaining ground beef, and form feet out of the meat.
Stick one stuffed leek roll upright in the 'ankle' area of each ground beef foot.
Use a knife and your fingers to form 'toes' at the front of each meat foot. Slide the pan into the oven and cook for 60 minutes.
Spread ketchup around the upright leeks and use the onion pieces you set aside earlier as toenails.