Pair two onion rings and insert mozzarella between layers, securing with toothpicks.
Mix flour, baking powder, and salt. Coat rings evenly in mixture.
Whisk milk and egg, then combine with leftover flour mix to form batter.
Dip rings in batter, let excess drip off, then coat in Panko. Repeat for double coating.
Heat oil to 350°F and fry until golden brown, about 2 minutes per side.
Drain on paper towels, remove toothpicks, and serve hot with marinara sauce.
