Place fish in a shallow dish. Squeeze juice of 1 lime over fillets.
Combine chili powder and next 3 ingredients. Sprinkle 1 1/2 Tbsp. seasoning mixture evenly over fish, coating both sides of fillets. Reserve remaining seasoning mixture.
Combine cornmeal and reserved seasoning mixture in a shallow dish. Dredge fish fillets in cornmeal mixture, shaking off excess.
Pour oil to a depth of 1 1/2 inches in a Dutch oven; heat to 350°. Fry fillets, in batches, 2 to 3 minutes or until golden brown. Drain fillets on wire racks over paper towels.
Break each fillet into chunks, using a fork. Place fish in warmed tortillas, and serve with Mango Salsa, Guacamole, and desired toppings. Garnish, if desired.
Cook's Notes: While we love the fried fish flavor, we also tried baking and grilling the flounder fillets. Both are great options. Here's how to do it:
Baked Flounder Fillets: Omit oil from Spicy Fish Tacos recipe. Assemble as directed. Spray cornmeal-coated fish with vegetable cooking spray. Bake at 350° on a lightly greased rack on a baking sheet 12 minutes or until fish flakes with a fork.
Grilled Flounder Fillets: Omit oil and cornmeal from Spicy Fish Tacos recipe. Assemble as directed. Reduce salt to 1 tsp. Sprinkle fish evenly with chili powder, salt, cumin, and red pepper. Lightly grease a large piece of heavy-duty aluminum foil; line cooking grate with foil. Place fish on lightly greased side of foil; grill, covered with grill lid, over high heat (400° to 500°) 8 minutes or until fish flakes with a fork, turning once.