Prepare the Roast Beef: Preheat oven to 220°C (425°F). Rub beef with olive oil, salt, pepper, Dijon mustard, thyme, and garlic.
Roast the Beef: Place in a roasting pan and cook for 15 minutes at 220°C, then lower temperature to 180°C (350°F) and roast for 40-50 minutes (for medium-rare).
Rest the Beef: Remove from oven, cover with foil, and let rest for 15 minutes before slicing.
Make Yorkshire Pudding Batter: Whisk flour, salt, egg, milk, and water in a bowl until smooth. Let batter rest while beef roasts.
Bake Yorkshire Puddings: Increase oven to 220°C (425°F). Pour beef drippings into a muffin tin and heat for 5 minutes. Pour batter into hot tins and bake for 20 minutes, until puffed and golden.
Prepare the Salad: Toss watercress with olive oil, red wine vinegar, salt, and pepper.
Serve: Slice roast beef and serve with Yorkshire puddings and watercress salad.