Preheat oven to 350° F
Combine the first six ingredients in a small bowl; stir in a grind of black pepper. Set tartar sauce aside
Combine onion and ice water; soak 20 minutes. Drain on paper towels
Sprinkle ¼ tsp each of salt and pepper evenly over the fish.
Place flour in a shallow dish; egg whites in another, and the bread crumbs in yet another. Dredge both sides of fish in flour; dip in egg white, and dredge this time in bread crumbs.
Heat a large nonstick skillet over medium-high heat, add oil; swirl to coat. Add fish; cook 2 minutes on each side or until browned.
Place fish on a baking sheet. Bake for 5-6 minutes, or until it flakes easily when tested with a fork.
On the buns, spread tartar sauce. Top each a fish fillet, onions, and tomatoes