Instructions: Rinse beef and pat dry. If tenderloin tapers, fold tip under to make roast uniformly thick. Tie crosswise at 2-inch intervals with cotton string to form a compact roll. Rub meat with oil and sprinkle with salt and pepper. Set on a rack in a shallow 10- by 15-inch pan.

Roast meat in a 425 degree oven until a thermometer inserted in center of thickest part reaches 120 degrees (very rare), about 40 minutes, or to your taste.

Transfer meat to a rimmed plate and let cool 20 minutes. Add meat juices and 1/4 cup water to roasting pan and scrape drippings free; pour into a small container, cover, and chill. Cover roast and chill until cold, at least 4 hours.

Meanwhile, rinse mushrooms, trim off and discard discolored stem ends, and mince. In a 10- to 12-inch frying pan over medium-high heat, frequently stir mushrooms, wine, butter, shallots, and nutmeg until liquid is evaporated and mushrooms are browned, 20 to 25 minutes. Let cool, about 15 minutes.

Unfold pastry on a lightly floured board; if there are 2 sheets, stack them. Cut a 1/2-inch-wide strip off 1 edge and reserve. Roll pastry into a neat rectangle 2 inches longer than the roast and 2 inches wider than its circumference.

Gently pat mushroom mixture over pastry up to 1 inch from edges. Pat beef dry and remove string. Center roast lengthwise on pastry. Brush pastry edges with water. Using both hands, quickly lift 1 long side over meat. Repeat to lift opposite side over meat; press edges together to seal. Fold in pastry ends, overlapping to cover ends of roast; press firmly to seal. Set roast seam down in an oiled rimmed 10- by 15-inch pan.

On floured board, roll reserved pastry strip 1/8 inch thick. With the tip of a sharp knife or flour-dusted cookie cutters, cut out decorative shapes, such as leaves. Brush a little water onto 1 side and arrange on roast, moistened-sides down; press gently to hold in place.

In a small bowl, beat egg yolk to blend with 1 tablespoon water. Brush mixture over pastry.

Bake Wellington in a 400 degree oven until pastry is richly browned, 25 to 30 minutes. With 2 wide spatulas, transfer roast to a platter. Let rest 15 to 20 minutes. Garnish with watercress. Cut roast crosswise into 1-inch-wide slices.

Serve with Cabernet Bearnaise Sauce and salt and pepper to taste.

This recipe yields 12 servings.

Comments: Up to 1 day ahead, roast meat and cook mushrooms; chill separately, airtight. Up to 4 hours ahead, wrap meat in pastry; cover and chill. Bake just before serving; meat reheats but doesnt cook further. To serve sauce hot, make while the roast rests; otherwise, make up to 2 hours ahead and let stand at room temperature.

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