In a shallow bowl combine 1.5 cups all-purpose flour, 0.25 cups cornstarch, 1 tsp baking powder, 1 tsp salt, 0.5 tsp black pepper, and 0.25 tsp cayenne (if using). Whisk to evenly distribute ingredients.

Make beer-style batter

In a separate mixing bowl lightly beat 1 large egg. Add 1 cup cold sparkling water and whisk briefly until just combined; do not overmix. Fold the wet mixture into the dry mixture until a thick, slightly lumpy batter forms. Adjust thickness: batter should coat the back of a spoon — add a tablespoon more flour if too thin or a splash more water if too thick.

Prepare breadcrumbs

Place 1 cup fine dry breadcrumbs (or panko) in a shallow dish for a final light coating. Set the breadcrumbs next to the batter station.

Fish & Seasoning

Portion and season

Pat 1 lb grouper fillets dry with paper towels and cut into individual portions (4–6 oz). Squeeze half a lemon over both sides of the fillets. Sprinkle each piece with 0.5 tsp salt (total) and 0.25 tsp black pepper (total) and 1 tsp Old Bay seasoning; press gently so seasoning adheres. Let sit at room temperature for 5–10 minutes while you heat the oil.

Dredge fish

Working one piece at a time: lightly dust the seasoned fillet with 1–2 tablespoons of plain flour from the batter dry mix (this helps the batter stick), then dip into the batter, letting excess drip back into the bowl. Immediately transfer to the breadcrumbs and press gently to pick up a thin layer of crumbs on the surface. Place coated pieces on a plate while you finish the rest.

Frying & Serving

Heat oil

Pour 6 cups neutral oil into a deep heavy-bottomed pot or deep fryer and heat to 350°F (175°C). If you don’t have a thermometer, heat until a small cube of bread browns in about 45–60 seconds.

Fry grouper

Fry the coated grouper in batches—do not overcrowd the pot. Carefully lower pieces into the hot oil and fry until golden brown and the internal temperature reaches 145°F (63°C), about 3–5 minutes per side depending on thickness. Use a slotted spoon or spider to turn pieces halfway through if needed. Maintain oil temperature between 325–350°F by adjusting heat.

Drain

Remove cooked grouper to a wire rack set over a baking sheet or to paper towels to drain briefly. Let rest 2 minutes to finish cooking internally and to crisp up.

Serve

Plate the grouper with lemon wedges and tartar sauce on the side. Sprinkle with 2 tbsp chopped parsley if desired. Serve immediately while hot and crispy.