Heat oven to 350°F. Melt margarine in 13x9-inch (3-quart) glass baking dish in oven. In shallow pan, combine flour and onion salt.
In medium bowl, combine egg, water and lemon juice. Place corn flakes in another medium bowl. Coat fish fillets with flour mixture; dip in egg mixture. Coat with corn flakes. Place fish in baking dish; turn to coat with melted margarine.
Bake at 350°F. for 15 to 20 minutes or until fish flakes easily with fork.