Trim any fat from meat. Brush meat evenly with Dijon-style mustard. Sprinkle meat with salt and black pepper. In a a shallow dish, combine rosemary, thyme and sage. Roll meat in the mixed herbs, pressing herbs on all sides of meat.
In a very large nonstick skillet, brown tenderloin in hot oil over medium-high heat, turning to brown all sides. Transfer tenderloin to a rack in a shallow roasting pan. Roast in a 425 degree F oven for 25 minutes or until an instant-read thermometer inserted in pork registers 155 degrees F. and juices run clear. Remove from oven; cover with foil and let stand 15 minutes (meat temperature will rise to 160 degrees F).
For currant sauce: In a small saucepan combine jelly, vinegar, and butter. Heat and stir until jelly is melted. Remove from heat. Stir in horseradish, lemon peel and juice.
To serve, line a serving platter with watercress. Slice pork and arrange on top of the watercress; spoon some of the currant sauce over pork. Serve remaining sauce on the side. Makes 4 servings.