Clean the mushroom caps by using a spoon to remove the gills. Remove or trim any overhanging margin, so that the cap is as flat as possible. 

Assemble a breading station with three plates or shallow bowls. In the first bowl, combine flour, garlic powder, 1 teaspoon salt, and ¼ teaspoon freshly ground pepper (about 10 turns of a peppermill).

In the second, combine eggs, a pinch of salt, and pepper to taste. In the third, combine panko crumbs, 1 teaspoon salt, and about ¼ teaspoon pepper. Dredge each mushroom in flour, shaking off excess, then egg, then panko.

Preheat oven to 350°F (180°C). Heat about ¼ inch of oil in a large frying pan and fry the mushrooms, gill-side down first, for about 5 minutes per side until golden brown. Remove mushrooms and drain on paper towels until ready to assemble.

Place half of the mushroom caps on the bottom of a 9×13-inch baking pan, cutting in half to fit, if necessary. Pour half the sauce over the mushrooms, then sprinkle with half the mozzarella.

Add another layer of mushrooms, then sauce and mozzarella, and finish with Parmesan cheese. Cover with parchment paper and foil, then bake for 30–40 minutes.