Preheat oven to 350°F.
Make mini meatballs: In a large bowl, combine ground beef, bread crumbs, garlic, parsley, and crushed red pepper flakes and season with salt and pepper. Roll into mini meatballs, about ½-1 inch in diameter and transfer to a parchment paper-lined baking sheet. Bake until browned and cooked through, 15 minutes.
Trim the outer crust from each slice of bread.
Tip: choose slices from the center of the loaf so they are larger in size.
Brush egg wash on the outer edge (about ½” wide) of six slices of bread.
Spoon mozzarella, marinara, and 3 mini meatballs onto each of the six bottom slices of bread, staying inside the egg wash rim. Be careful not to overstuff.
Gently compact the six remaining slices of bread using your hands, then place one over each pocket sandwich. The edges of the bread should overlap the bottom slice of bread with the egg wash.
Gently pinch the top and bottom slices of bread together to seal the sandwiches, then crimp the edges with a fork.
Brush the tops of the sandwiches with more egg wash and transfer to a prepared baking sheet.
Bake for 5-10 minutes or until golden brown.
Top with fresh parsley and serve.
Recipe adapted from: www.delish.com