Preheat your oven to 350F
Chop up your bread slices and place them in the milk. Let soak until milk is absorbed
Mix the milk soaked bread, parsley, eggs, parmesan, nutmeg, salt, pepper and olive oil together in a large bowl. Add in your combined ground meats. Mix together until just mixed.
Form the meatballs into partial balls, leaving an indent. Place the cubed mozza in the indent. Form the meatball to encase the cheese.
Place the formed meatballs on a parchment lined baking sheet
Bake the meatballs once formed in a 350F oven for about 15 minutes until cooked through.
In an oven proof dish lay a pool over marinara sauce. Put the meatballs on top. Drizzle the meatballs with a bit of the sauce and dot the pan with a bit more cheese.
Bake under the broiler for a couple minutes or so until cheese is melty and browned.