This Italian-style burger has all your favorite parts of a meatball, is stuffed with mozzarella AND topped with melty cheese. To round it out we serve it on a garlic bread bun with crispy pancetta and marinara sauce.
Mix the meatball mix, parsley, Parmesan, fennel, 2 of the garlic cloves, 2 teaspoons salt and a generous amount of pepper in a large bowl. Divide the mixture into 4 equal balls. Working with one ball at a time, divide it in half and flatten to make 2 patties about 5 inches in diameter. Fold 2 slices of mozzarella into quarters and place into the center of the patty. Top with the other patty and press the meat together very tightly, forming a large patty shape with the cheese secured inside. Repeat with the remaining balls and 6 of the cheese slices. (At this point, you can freeze the burgers. Thaw in the refrigerator before cooking.)
Cook the pancetta in a large cast-iron skillet over medium-high heat, flipping once, until the fat has released and it's crispy, 3 to 4 minutes. Remove to a paper-towel-lined plate. Add the burgers to the pan and cook, flipping once and topping each patty with 1 slice of the remaining cheese with 1 minute of cooking time left. Cook until nicely golden brown and an instant-read thermometer registers 150 degrees F for medium, 4 to 5 minutes per side.
Meanwhile, melt the butter in a small saucepan or microwave-safe bowl and stir in the remaining garlic. Place the hamburger buns onto a baking sheet and brush both sides with the garlic butter. Broil until toasted, 1 to 2 minutes. Spread 1 to 2 tablespoons marinara sauce on each bottom half of the buns. Place a burger patty on top of the bottom half of the bun. Then top each burger with a spoonful of marinara, 2 pieces of pancetta, a sprinkle of Parmesan and the top half of the bun.