Cut fish in 3 inch pieces. Blanch vegetables. Cover bottom of pan with butter, vegetables and water. Over this place the fish and simmer slowly until tender. Carefully lift out fish and vegetables. Strain liquid and add to the creamed butter and flour to make sauce. When smooth, add 3 well beaten egg yolks. Heat but do not boil. Arrange fish in center of platter and cover with sauce. Garnish with vegetables.