1. Preheat grill to 350° to 400° (medium-high) heat. Rub steaks with salt and pepper; let stand 10 minutes.
2. Meanwhile, stir together butter, mustard, and tarragon.
3. Grill steaks, covered with grill lid, 8 to 10 minutes on each side or to desired degree of doneness. Remove from grill, and spread with tarragon butter. Serve with Mushroom Gnocchi.