Steam the whitefish your preferred way, I use my rice cooker for this. For a rice cooker, simply add 1-2 cups of water to the pot, place the steamer tray on top and cover with the lid. Press cook, when the water is boiling, remove the lid and place fish in the steamer tray and cover with the lid.
Steam until cooked through, about 5-10 minutes.
Remove fish, drain, flake apart meat, remove any bones or skin.
Melt butter in a medium saucepan, stir in flour until smooth, slowly stir in milk, cook and stir until thickened.
Add a small amount of sauce to the egg yolks and stir together, then add all the egg yolks to the sauce and stir in.
Add the fish, onion, and lemon juice. Heat through.
Serve over toast points, enjoy.