Rub fillets with tarragon and pepper. Melt 2 tablespoons butter in a large skillet over medium-high heat. Cook fillets 4 to 5 minutes on each side or to desired degree of doneness. Remove from skillet, and keep warm.
Melt remaining 3 tablespoons butter in skillet. Add mushrooms and sautÃ© 3 to 4 minutes or until tender. Add wine (or beef broth) and cook 1 to 2 minutes, stirring to loosen particles from bottom of skillet. Stir in sour cream. Sprinkle 1/4 cup cheese into sauce, stirring until melted.
Arrange fillets on a serving platter, and drizzle with sauce. Sprinkle with remaining cheese, and garnish, if desired.