Preheat oven to 375°F. In a small bowl, stir together milk and panko and set aside to allow the breadcrumbs to absorb the milk.

In a large mixing bowl, combine sausage, ground beef, Italian seasoning, eggs, garlic powder, salt, pepper, and parmesan cheese. Add the breadcrumb mixture and combine all ingredients well.

Line a mixing bowl with plastic wrap, leaving lengths of plastic wrap in each direction so the meatball can be shaped by folding up the plastic wrap.

Press 2⁄3 of the meatball mixture into the bowl, pressing it evenly across the bottom and up the sides of the bowl to the rim. The meat mixture should be at least 1⁄2-3⁄4” thick.

Sprinkle grated mozzarella into the bottom of the meat-lined bowl.

Mix together the cooked spaghetti and about 1⁄2 the jar of marinara sauce.

Mound the spaghetti over the cheese in the meatball bowl.

Press the remaining 1⁄3 meatball mixture over the spaghetti, pinching it around the sides where it meets with the meat mixture lining the bowl.

Fold the tails of the plastic wrap taut over the meat to contain the meatball fully. Turn out of the bowl and gently press and shape the meatball with your hands so that it is evenly shaped and fully sealed.

Place the formed meatball back into the bowl and then unwrap the top of the plastic wrap.

Invert the meatball onto a parchment-lined baking sheet and remove the plastic wrap completely.

Bake for 40-45 minutes or until the internal temperature of the meat reaches at least 145°F and the top of the meatball is browned.

Transfer meatball to a serving platter and top with the remaining marinara sauce. Sprinkle with grated parmesan and/or chopped parsley, if desired.