Wash and dry mushrooms; chop.
Saute" mushroom and onion in 2 tablespoons butter for 5 minutes.
Melt additional 2 tablespoons butter; stir in flour.
Add milk; cook, stirring, until slightly thickened.
Add beaten egg yolks.
Remove from heat and fold in stiffly beaten egg whites.
Pour into well-buttered and floured 4-6 cup ring mold.
Place mold in a pan with 1 inch of water.
Steam in a 350° oven for 30 to 40 minutes.
Unmold and fill with creamed chicken or buttered vegetables.