Slice the onion into 1/2-inch rings and separate them, discarding the small inner rings. Fill each ring with shredded mozzarella and press the cheese lightly to secure it.
Set up three shallow bowls. Bowl 1: Mix flour, garlic powder, onion powder, paprika, salt, and pepper. Bowl 2: Beat the eggs. Bowl 3: Spread the panko breadcrumbs.
Coat each onion ring by dipping it into the flour mixture, followed by the beaten eggs, then the panko breadcrumbs. Press lightly to adhere. Refrigerate for 15-20 minutes.
Heat oil to 350°F (175°C) in a deep skillet. Fry the onion rings in batches for 3–4 minutes per side until golden and crispy. Remove and drain on paper towels.
Melt butter in a small saucepan over low heat. Stir in garlic, parsley, lemon juice, red pepper flakes (optional), and salt. Cook for 1-2 minutes until fragrant, then set aside.
Arrange the onion rings on a platter. Drizzle with garlic butter or serve on the side. Garnish with parsley and serve hot and gooey.