Wash halibut steak and pat dry.
Brush bottom of baking pan with melted butter, sprinkle with chopped onion.
Place steak on top.
Pour egg yolk over fish.
Sprinkle with salt and pepper and add lemon juice.
Pat on remaining butter.
Bake in a 400° oven for 30 minutes.
Place fish on a hot platter.
Make a sauce in the pan with drippings combined with flour and water.
Heat and blend until smooth.