Peel the onions and slice into 1/4- to 1/2-inch thick rings. Gently separate into individual rings and set aside. For best results use 2 large onions to yield about 4–6 servings.

Make batter

In a medium bowl whisk together 1.5 cups all-purpose flour, 1/2 cup cornstarch, 1 tsp baking powder, 1 tsp kosher salt, 1/2 tsp black pepper, 1/2 tsp garlic powder, and 1/2 tsp paprika. In a separate small bowl beat 1 large egg with 1 cup cold club soda (or cold beer). Pour the wet mixture into the dry ingredients and whisk briefly until just combined — batter should be thick but pourable and slightly lumpy. If using panko for extra crunch, place 1 cup panko in a shallow bowl for dredging.

Heat oil

Pour enough vegetable oil into a heavy-bottomed pot or deep fryer to reach about 2–3 inches deep (about 1 liter for a medium pot). Heat oil to 350°F (175°C). If you don't have a thermometer, test by dropping a small bit of batter — it should sizzle and rise to the surface within a few seconds.

Coat and fry

Working in small batches to avoid crowding, pat onion rings dry with paper towels. Dip each ring into the batter, letting excess drip back into bowl, then optionally dredge in panko for an extra-crisp coating. Carefully lower rings into hot oil with a slotted spoon or spider. Fry until golden brown and crisp, about 2–3 minutes per side, flipping if necessary. Do not overcrowd the fryer — 4–6 rings per batch depending on pot size.

Drain and season

Use a slotted spoon to transfer fried rings to a wire rack set over a baking sheet, or paper towels for a quicker clean-up. Immediately season lightly with a pinch of salt while still hot. Repeat with remaining rings, keeping cooked rings in a warm oven (200°F/95°C) if making ahead so they stay crisp.

Boom Boom Sauce

In a small bowl combine 1/2 cup mayonnaise, 2 tablespoons gochujang (or Sriracha), 1 tablespoon sweet chili sauce, 1 teaspoon hot sauce (optional), 1 teaspoon rice vinegar or lime juice, 1 teaspoon honey or sugar, 1/4 tsp garlic powder, 1/4 tsp smoked paprika, 1/4 tsp salt, and 1/8 tsp freshly ground black pepper. Whisk until smooth and taste — adjust heat or sweetness as desired. Chill in the fridge for at least 15 minutes to let flavors meld. Serve cold alongside hot onion rings for dipping.