Place the flour and eggs in separate bowls. Combine the Parmigiano-Reggiano and panko crumbs in another bowl and set aside. Separate the onion rings, then dip into the flour, followed by the egg, then the panko crumbs. Place in the freezer for 15–20 minutes. In a large saucepan or deep-fryer, heat the oil to 165°C. Deep-fry the onion rings in batches until golden, using a tea strainer to scoop out any bits left in the oil between batches. Drain on paper towels and season with salt. Combine the mayonnaise and horseradish and place in a small serving dish. Serve the onion rings on a round platter with the Horseradish Mayo placed in a bowl in the centre. SIMON SAYS: You will find fresh horseradish in good vegetable shops when in season. Once grated, use horseradish immediately or mix it with vinegar, otherwise it loses its pungency and becomes bitter.