Preheat oven to 425°. Place fish in a greased 13-in. x 9-in. baking dish; sprinkle with pepper. Beat egg white with salt if desired until stiff peaks form. Fold in mayonnaise, dill and onion; spoon over fish. Bake, uncovered, until topping is puffed and fish flakes easily with a fork, 15-20 minutes. Garnish with dill and lemon if desired.