Position an oven rack to the middle position and preheat the oven to 350 degrees F. Put a cooling rack in a rimmed baking sheet.
Slice off the top 2 inches of the bread boule and reserve. Hollow out the inside, taking care to leave a 1/2-inch-thick shell on all sides. Tear enough of the bread scraps to make about 1/2 cup and put them in to a large bowl along with the milk. Allow it to sit for 5 minutes. (Save the remaining bread scraps for another use.) Add the parsley, egg, 1/4 cup of the Parmesan, 1/4 teaspoon of the crushed red pepper, half the garlic and 1 teaspoon salt. Use your hands to stir everything together until well combined. Add the beef and mix until just combined.
Drop leveled tablespoons of the meat mixture onto the rack in the baking sheet. Use slightly damp hands to form the mounds into balls. Bake until slightly browned and just cooked through, about 10 minutes.
Spread 1/2 cup of the marinara sauce in the bottom of the boule and pour the remaining 1 1/2 cups of the sauce in a second large bowl. Add the meatballs to the bowl and stir gently to combine.
Wipe the rack clean, return it to the baking sheet and put the boule on it. Fill the boule in this order: Sprinkle the inside with 1 cup of the mozzarella, then add half the sauce-covered meatballs and 1/4 cup of the Parmesan; repeat the layers of mozzarella, meatballs and Parmesan. Top with the remaining 1 cup mozzarella.
Whisk together the butter, remaining garlic and crushed red pepper and 1/2 teaspoon salt. Generously brush the underside of the bread top with the butter mixture and put it on top of the bread bowl. Generously brush the outside of the loaf with the remaining butter mixture. Weigh down the bread bowl with a large cast-iron skillet. Alternatively, you can use an aluminum skillet and weighed down with a couple of large cans. Allow to sit at room temperature for 30 minutes.
Remove the skillet from the bread bowl and sprinkle the remaining 1/4 cup Parmesan on top. Bake, rotating the baking sheet halfway through, until the bread is crispy and the cheese is melted and golden brown, 35 to 40 minutes. Let cool for 10 minutes before slicing.
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