Combine flour, paprika, cayenne, and baking powder in a medium bowl until uniformly blended.

Whisk milk with the beaten egg in a small bowl and set aside.

Stir together Panko, plain bread crumbs, and Parmesan in a third bowl so the coating is evenly mixed.

Coat onion rings in the seasoned flour, immerse each piece in the egg mixture, press into the crumb blend, return to the egg bath, then coat once more in crumbs before arranging on a tray.

Pour oil into a heavy pot or deep fryer to a depth of about 5 cm and heat to 165 °C (325 °F).

Fry prepared rings in batches for 2 – 3 minutes, turning midway, until an even golden hue develops.

Transfer fried rings to a rack set over a tray, allowing excess oil to drip away.

For the dip, stir mayonnaise, Sriracha, and sugar until smooth.

Present onion rings hot, garnished with parsley if desired, alongside the Sriracha sauce.