Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large mixing bowl, combine the ground beef, egg, gluten-free breadcrumbs, 1/4 cup Parmesan, onion, garlic, Italian seasoning, salt, black pepper, and 1/4 cup marinara. Mix gently with your hands or a fork just until combined.
Divide the mixture evenly among the 8 greased muffin cups. Press it in lightly without packing it tightly.
Spoon the remaining 1/4 cup marinara over the tops, then sprinkle with the remaining 2 tablespoons Parmesan.
Bake until the muffins are cooked through and no longer pink in the center, about 20 to 25 minutes. Let them rest in the pan for 5 minutes before carefully removing and serving.