Preheat oven to 450°.
Combine first 4 ingredients in a shallow dish. Combine breadcrumbs and next 3 ingredients in another shallow dish.
Dip fish fillets in egg white mixture. Dredge fish in breadcrumb mixture, pressing firmly to coat. Place fish on a wire rack; let stand 10 minutes. Place jelly-roll pan in oven to heat while fish stands.
Coat fish well with cooking spray. Remove hot pan from oven; coat with cooking spray. Place fish on pan in a single layer. Bake at 450° for 25 minutes or until fish flakes easily when tested with a fork.
While fish bakes, combine mayonnaise and mustard. Spread mayonnaise mixture evenly on 1 side of each hoagie roll; top each roll with 1 lettuce leaf, 1 catfish fillet, and 2 tomato slices. Top with remaining bread.