Pat the grouper fillets dry with paper towels. Cut into 4 equal portions (about 4 oz each) if not already portioned. Lightly season both sides with 1 tsp fine sea salt and 1/2 tsp freshly ground black pepper.

Set up dredge and batter

Place 1/2 cup all-purpose flour on a shallow plate for dredging. In a chilled bowl, whisk together 1/4 cup cornstarch, 1/2 tsp baking powder and the cold egg. Slowly whisk in 3/4 cup ice-cold club soda until mostly smooth; a few small lumps are fine. Keep the batter very cold—set the bowl in a larger bowl filled with ice if desired—to ensure a light, crisp tempura-style coating.

Heat oil

Pour about 4 cups vegetable oil into a deep skillet or Dutch oven (oil depth ~2 inches). Heat over medium-high until it reaches 350°F (175°C). If you don't have a thermometer, test with a small drop of batter: it should sizzle and float immediately.

Dredge and fry

Lightly dredge each seasoned fillet in the 1/2 cup all-purpose flour, shaking off excess. Dip into the cold batter to coat fully, allowing excess to drip off. Carefully lower into the hot oil, frying 2–3 pieces at a time so oil temperature remains steady. Fry until golden and crisp, about 3 to 4 minutes per piece depending on thickness, turning once. The internal temperature should reach 145°F (63°C) or the flesh should flake easily. Transfer to a wire rack set over a baking sheet or paper towels to drain while you finish the rest; keep warm in a 200°F (95°C) oven if needed.

Firecracker Sauce

Make the sauce base

In a small saucepan, heat 1 tablespoon vegetable oil over medium heat. Add 2 cloves minced garlic and 1 teaspoon minced fresh ginger; sauté 30 seconds until fragrant but not browned. Add 1/2 teaspoon red chili flakes and the finely chopped Thai red chile; cook 15–30 seconds more to bloom the heat.

Combine sweet-sour-spicy elements

Add 3 tablespoons rice vinegar, 3 tablespoons ketchup, 2 tablespoons honey, 1 tablespoon soy sauce, 1 teaspoon sriracha, and 3 tablespoons water to the pan. Stir to combine and bring to a gentle simmer so flavors meld, 1–2 minutes.

Thicken sauce

Mix 1 teaspoon cornstarch with 1 tablespoon cold water to make a slurry. Slowly whisk the slurry into the simmering sauce. Cook 1–2 minutes more until the sauce thickens and becomes glossy. Remove from heat and stir in 1/2 teaspoon toasted sesame oil. Taste and adjust: add more honey for sweetness, vinegar for tang, or sriracha for heat.

Finish

Stir in sliced green onions (reserve a few for garnish). If the sauce is too thick, thin with up to 1 tablespoon water to reach a glossy, pourable consistency. Keep warm until ready to serve.

Assembly and Serving

Place a bed of shredded cabbage or mixed greens on each serving plate (optional). Arrange tempura-fried grouper pieces on top.

Spoon the warm firecracker sauce over the fried grouper, coating each piece generously but leaving some crisp edges visible. Garnish with remaining sliced green onions, a sprinkle of toasted sesame seeds if using, and lemon or lime wedges on the side.

Serve immediately while the fish is hot and crisp. Provide extra sauce on the side if desired.