Preheat oven: Preheat your oven to 375°F (190°C) and lightly grease a muffin tin or a baking sheet. Make the meat mixture: In a large bowl, combine the ground beef, breadcrumbs, Parmesan, milk, egg, chopped onion, minced garlic, Italian seasoning, salt, and pepper. Mix everything until just combined. Prepare the cheese filling: In a small bowl, mix together the mozzarella, cream cheese, cheddar, and parsley until well combined. You can also add a pinch of salt and pepper to taste. Form the meatloaf bites: Take a small portion of the meat mixture and roll it into a ball, then create a small indentation in the middle with your finger. Add about 1 tablespoon of the cheese mixture into the center, then wrap the meat around the cheese and roll it back into a ball. Repeat for the rest of the mixture. Bake: Place the stuffed meatballs in the muffin tin or on a baking sheet. Bake for 20-25 minutes, or until the meatloaf bites are cooked through and the cheese inside is melted. (You can check with a meat thermometer to make sure they reach 160°F internal temperature.) Topping (optional): If desired, spoon a little marinara sauce on top of each meatloaf bite and sprinkle extra mozzarella cheese on top. Return to the oven for another 5-7 minutes to melt the cheese and heat the sauce. Serve: Let the meatloaf bites cool slightly before serving. Enjoy as an appetizer or with a side salad, mashed potatoes, or veggies!