Preheat oven to 425 degrees Fahrenheit.

Place fish in a greased 9×13-inch (23×33-cm) baking dish (glass is preferred) and sprinkle with pepper.

Beat egg whites with salt until stiff peaks form.

Fold in mayonnaise, dill, and onion.

Spoon over fish and spread evenly.

Bake uncovered for 10–20 minutes or until topping is puffed and fish flakes easily with a fork.

Add garnishes, if desired.