Sauté: Melt butter in a large pan and sauté the mushrooms and onions until soft.
Roux: Sprinkle the flour over the vegetables and stir for 2 minutes to cook out the raw flour taste.
Thicken: Gradually whisk in the chicken broth and half-and-half, stirring constantly until the sauce thickens into a glossy, smooth cream.
Combine: Add the cooked chicken, peas, and pimentos.
Simmer gently until everything is heated through.
Finish: Stir in the sherry and seasonings.
Serve: Traditionally served over toast points, puff pastry shells, or buttered biscuits.