Beef Fillets with Portobello Sauce

6 (6-ounce) beef tenderloin fillets
2 teaspoons chopped fresh tarragon
1/2 teaspoon freshly ground pepper
5 tablespoons butter or margarine, divided
8 ounces portobello mushroom caps, sliced
1/3 cup dry red wine or beef broth
1/2 cup sour cream
3 ounces Stilton or blue cheese, crumbled and divided
Garnish: fresh tarragon sprigs

1. Rub fillets with tarragon and pepper.
2. 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.
3. Melt remaining 3 tablespoons butter in skillet. Add mushrooms,
   and sauté 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.
4. Arrange fillets on a serving platter, and drizzle with sauce.
   Sprinkle with remaining cheese, and garnish, if desired.