Place tenderloin on rack in shallow roasting pan, tucking a small end of meat underneath. Brush with olive oil. Insert a meat thermometer, making sure it does not touch fat. Bake, uncovered at 425ºF. for 30 minutes or until thermometer registers 140ºF. (rare), 150ºF. (medium rare), or 160ºF. (medium). Slice into thin medallions and serve with Béarnaise Sauce; Garnish platter with watercress. Yield 10 - 12 servings.

Béarnaise Sauce:
1/2 cup of dry white wine
1 tablespoon of cider vinegar
2 teaspoons of chopped shallots
1 teaspoon of freshly ground pepper
2 teaspoons of chopped fresh chervil or  3/4 teaspoon dried
2 teaspoons of chopped fresh tarragon or 3/4 teaspoon of dried
1/2 cup of butter, cut into 1 inch chunks
6 egg yolks
2 teaspoons of chopped fresh tarragon or 3/4 teaspoon  dried
1/8 teaspoon of salt
1/8 teaspoon of red pepper

Combine the first 6 ingredients in a small saucepan, bring to a boil over medium heat. Reduce the heat to low, and simmer until half the liquid evaporates. Pour mixture through a strainer, reserving liquid, discard solids. Return liquid to saucepan and place over low heat. Alternately add butter and egg yolks to liquid mixture, stirring constantly. Stir in remaining spices. Yields 1 cup.