Thaw fish, if frozen. Rinse fish; pat dry with paper towels. If necessary, cut fish into 4 serving-size pieces. For sauce, stir together yogurt, cheese, and tarragon. Set aside.
Preheat broiler. Place fish on the greased unheated rack of a broiler pan. Sprinkle fish with salt and black pepper. Broil 4 inches from the heat for 6 to 9 minutes or until fish flakes easily when tested with a fork. Spoon sauce over fish steaks. Broil 30 to 60 seconds more or until heated through and cheese starts to melt. Makes 4 servings.