1. Trim excess surface fat from steaks. Rinse steaks and pat dry.
2. In a small bowl, mix wine, mustard, shallots, olive oil, and tarragon. Spread mixture thickly on all sides of steaks and stack them on a plate. Let stand at room temperature for at least 30 minutes, or cover and chill up to 1 day.
3. Lay steaks on an oiled barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Cook steaks, turning once, until browned on both sides and done to your liking in center of thickest part (cut to test), 10 to 13 minutes for medium-rare.
4. Transfer steaks to plates. Let rest in a warm place for 3 to 5 minutes. Garnish each with 1/2 cup watercress. Add salt and pepper to taste.