Melt 1/4 cup plus 2 tablespoons butter in top of a double boiler over simmering water; add flour, and stir with a wire whisk until smooth. Cook 1 minute, stirring constantly. Gradually add half-and-half and milk. Cook over simmering water, stirring constantly, until thickened and bubbly.
Sauté mushrooms in remaining butter in a small skillet until tender. Stir sautéed mushrooms into sauce. Add seafood, chopped egg, salt, pepper, lemon rind, and sugar; stir well. Cook over simmering water 25 minutes, stirring occasionally.
Add sherry; cook 5 minutes. Pour soup into individual serving bowls; garnish with grated egg yolk and chives, if desired.