Preheat the oven to 400 degrees Fahrenheit. Open the foil on both loaves of frozen garlic bread and set them cut side up on a baking sheet. Bake for 10 minutes, until the edges turn golden and the kitchen smells of garlic butter, then let cool for 5 minutes.

Tear the warm garlic bread into rough 2-inch pieces with your hands, keeping some larger chunks for texture. Scatter the pieces across the bottom of a 9x13-inch white ceramic baking dish, pressing lightly so the whole bottom is covered with no bare patches.

Lower the oven to 375 degrees Fahrenheit. Nestle the 24 thawed meatballs evenly into the gaps between the bread pieces, so every square you cut later gets at least one.

Pour the jar of marinara sauce over the bread and meatballs. Use the back of a spoon to push sauce down into the gaps so the bread underneath gets some too, not just the top.

Scatter the mozzarella and provolone evenly over the top, then sprinkle with the Parmesan and the red pepper flakes if using.

Cover the dish loosely with foil, shiny side down, and bake for 20 minutes, until you see sauce bubbling at the edges of the dish.

Remove the foil and bake, uncovered, for 10 to 15 minutes more, until the cheese melts completely and turns golden and blistered in spots.

Pull the dish out and let the casserole rest on the counter for 10 minutes, uncovered. This gives the cheese time to set so the squares hold their shape instead of sliding apart.

Scatter the chopped parsley over the top, cut into 12 squares directly in the dish, and serve warm.