1. Sprinkle fillets evenly with salt and 1/4 tsp. Old Bay seasoning. Arrange fillets on a lightly greased rack in an aluminum foil-lined broiler pan.
2. Bake at 425° for 20 to 25 minutes or until fish flakes with a fork and is opaque throughout. Finely chop fish.
3. Cook diced onion, bell pepper, and celery in 1/2 Tbsp. hot oil in a small skillet over medium-high heat, stirring often, 3 to 4 minutes or until tender.
4. Cook beer in a small saucepan over medium-low heat 5 minutes or until a thermometer registers 105° to 115°. Remove from heat; add yeast, and let stand 5 minutes. Transfer yeast mixture to a large bowl; stir in vegetables, milk, flour, hot sauce, catfish, remaining 1 Tbsp. oil, and remaining 1 tsp. Old Bay seasoning.
5. Cover and let rise in a warm place (85°), free from drafts, 30 minutes.
6. Beat egg whites at medium speed with an electric mixer until soft peaks form. Fold into catfish mixture.
7. Pour vegetable oil to a depth of 2 inches into a Dutch oven or electric deep-fat fryer; heat to 360°. Drop batter, by heaping tablespoonfuls, into hot oil; fry, in batches, 3 to 4 minutes or until golden brown, turning once. Drain on a wire rack over paper towels. Serve with tartar sauce and, if desired, lemon wedges.
Note: Keep beignets warm in a 225° oven for up to 30 minutes.