egg yolk, beaten to blend
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. Serves 4.