Another recipe from Joy of Cooking.
1/2 lb. mushrooms, sliced
2 tablespoons butter
2 cups chicken or vegetable stock
1/2 cup chopped celery
1 small onion, diced
2 tablespoons coarsely chopped parsley
Bechemel (see below)
salt to taste
1/8 teaspoon paprika
Saute mushrooms in butter until lightly browned. Add remaining ingredients and simmer 20 minutes. Blend. An immersion blender works well, but a standing blender or food processor works too.
Meanwhile make the bechemel:
2 Tbsp. butter
2 Tbsp. flour
1 cup milk
1 small onion
1/2 bay leaf
In an ovenproof saucepan melt 2 tablespoons butter. Stir in 2 tablespoons flour. Slowly add 1 cup milk, stirring well. Add 1 small onion studded with 2-3 whole cloves and 1/2 small bay leaf.
Stir until the sauce thickens. Place in a 350F oven, uncovered for 20 minutes.
Strain the sauce and stir into the blended mushroom soup mixture. Heat slowly, but do not boil. Season with salt and paprika.