Place reserved pan liquid in a medium saucepan; bring to a boil. Cook until liquid is reduced to 1 cup; set aside.
Melt 1/4 cup butter in a medium saucepan; stir in flour, salt, and red pepper. Add pan liquid. Cook over medium heat until sauce thickens. Add milk and whipping cream, stirring well. Continue cooking over medium heat until mixture begins to boil. Reduce heat, and simmer 2 to 3 minutes, stirring occasionally. Set aside.
Melt remaining butter in a small skillet; add mushrooms, 1/8 teaspoon pepper, and lemon juice. Sauté 5 minutes; drain. Add mushrooms to prepared white sauce, mixing well.