Step 1: Prepare the onions and mozzarella. Peel the onions and slice them into 1/2-inch thick rings. Carefully separate the rings. Choose the larger, more uniform rings for stuffing. Cut the mozzarella block into strips or small cubes that will fit snugly inside the onion rings.
Step 2: Stuff the onion rings. Take a larger onion ring and place a smaller onion ring inside it, creating a pocket. Stuff the mozzarella pieces between the two onion rings. Press gently to secure. Repeat with all desired rings. Place stuffed rings on a baking sheet lined with parchment paper and freeze for at least 20-30 minutes. This helps prevent cheese leakage.
Step 3: Set up your dredging station. In one shallow bowl, combine 1 cup flour, garlic powder, paprika, cayenne pepper (if using), salt, and pepper. In a second shallow bowl, whisk the eggs and milk. In a third shallow bowl, add the panko breadcrumbs.
Step 4: Dredge the stuffed onion rings. Remove the frozen stuffed rings from the freezer. First, dredge each ring in the seasoned flour, ensuring it’s fully coated. Shake off excess. Next, dip it into the egg wash, allowing any excess to drip off. Finally, thoroughly coat the ring in the panko breadcrumbs, pressing gently to make sure the panko adheres well. Place the coated rings back on the parchment-lined baking sheet. For extra crispiness, you can freeze them for another 10-15 minutes at this stage.
Step 5: Fry the onion rings. Heat about 2-3 inches of vegetable oil in a large heavy-bottomed pot or deep fryer to 350°F (175°C). Carefully lower 2-3 stuffed onion rings into the hot oil, being careful not to overcrowd the pot. Fry for 3-5 minutes, flipping occasionally, until golden brown and crispy. The cheese should be melted and gooey.
Step 6: Drain and serve. Remove the cooked onion rings with a slotted spoon and place them on a wire rack set over paper towels to drain excess oil. Season immediately with a sprinkle of salt. Serve hot with your favorite dipping sauce like marinara, ranch, or a spicy aioli.