To make horseradish creme, in a food processor bowl or blender container, combine cottage cheese, buttermilk, horseradish, lemon juice, and mustard. Process or blend until smooth. Cover and chill.

Trim all fat and silver skin from beef. Using a very sharp knife, cut tenderloin across the grain into 1/4-inch-thick slices. Gently flatten slices with the flat side of a meat mallet until they're about 1/8 inch thick. Cover and store in refrigerator until just before serving time.

Grill meat on rack of uncovered grill directly over hot coals, allowing 1 to 2 minutes on each side for medium rare (145 degree F). (Or coat a small skillet with non-stick cooking spray. Cook over medium-high heat for 1 to 2 minutes on each side.)

Serve meat with a generous dollop of horseradish creme. Garnish with watercress or chives. Makes 6 servings.