2.Sprinkle pepper over a sheet of baking paper; roll beef in the pepper.
3.Heat oil in medium shallow flameproof baking dish over high heat; add beef, turn until browned all over. Transfer dish to oven; roast beef 20 minutes, turning after 10 minutes for medium-rare, or until cooked to your liking.
4.Remove beef from dish to plate; cover tightly with foil. Cool.
5.Spread herbs over sheet of baking paper. Brush beef with mustard; roll beef firmly in herb mixture. Slice beef thinly.
6.Split rolls in half almost all the way through. Spread butter, then extra mustard inside rolls; fill rolls with beef and watercress.
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