Place the onions in a small bowl and cover with cold water. Let soak for 15 minutes. Drain.
Meanwhile, using a small knife and your hands, remove the skin, bones, and any brown bits from the whitefish and discard. Flake the whitefish into small pieces into a medium bowl, checking again for any bones.
Add the egg and onion to the whitefish and mash together with a fork. Stir in the mayonnaise and lemon juice. (For a smoother textured salad, pulse the whitefish mixture in a food processor.) Season with salt and pepper to taste. Serve immediately or store in the refrigerator for up to 3 days.
If you're buying cleaned whitefish, you'll need 3 1/3 cups.
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