Place tenderloin on a foil-lined baking sheet or roasting pan. Make 12 small slits on top side of meat; insert garlic halves.
Combine mustard and 2 tablespoons prepared horseradish in a small bowl, and spread evenly over top and sides of beef.
Bake at 350 for 25 to 30 minutes or until desired degree of doneness. (For rare, remove from oven at 13
Beef will continue to cook to 140 with residual heat.) Cover and chill several hours or overnight.
Combine sour cream and remaining 2 tablespoons horseradish; serve with chilled beef.
For individual canaps, slice beef 1/4 inch thick, and cut into wedges.
Place on toasted French baguette slices, top with a small dollop of sour cream mixture, and garnish with fresh parsley. Makes about 5 dozen hors d'oeuvres.
