Beef eye of round may be substituted for the beef tenderloin. Follow the same method, roasting 10 minutes more for medium rare (30 to 35 minutes total).
Preheat oven to 425 degrees. Heat oil in a large ovenproof skillet over medium-high heat. Generously season beef with salt and pepper. Add to skillet, and brown on all sides, about 10 minutes total. Transfer skillet to oven.
Roast until beef registers 130 for medium-rare, 20 to 25 minutes. Let beef stand for 20 minutes. Cut into 1/4-inch-thick slices.
Meanwhile, mash garlic with a fork in a small bowl. Stir in mayonnaise, lemon juice, and herbs. Season with salt and pepper. (Garlic-herb mayonnaise can be stored in the refrigerator for up to 1 day.)
Spread 1 tablespoon of the mayonnaise onto each bread slice. Top with beef. Just before serving, add watercress. Season with salt and pepper.