Melt butter in heavy large saucepan over medium heat. Add mushrooms and saute until soft, about 4 minutes. Add flour and stir for 2 minutes. Slowly mix in Half and Half. Simmer until thickened, stirring frequently, about 4 minutes. Combine yolk and lemon juice in small bowl. Gradually whisk in half of soup. Return to saucepan and stir until heated through; do not boil. Season generously with salt and pepper.

Recipe from "Too Busy To Cook", Bon Appetit, December, 1989.