If using fresh mushrooms, wipe clean; do not wash. Trim and slice them. Saute over high heat in 2 Tbsp. butter for 5 minutes, stirring often. After 5 minutes, scatter parsley and sage over mushrooms and season with salt and pepper.
Melt remaining butter and stir in flour, egg yolk, and lemon juice. Pour sauce over mushrooms. Turn off heat.
Stir well and place over low heat for a few minutes to thicken. Don't let sauce boil. Transfer to serving dish. Serve immediately.