Peppered Beef Tenderloin
["1 (8 oz.) carton sour cream", "3 Tbsp. Dijon mustard", "2 Tbsp. prepared horseradish", "2 Tbsp. whole green peppercorns", "2 Tbsp. whole red peppercorns", "2 tsp. coarse salt", "1 (4 to 4 1/2 lb.) beef tenderloin, trimmed", "1 c. chopped fresh flat-leaf parsley", "1/4 c. butter, softened", "3 Tbsp. Dijon mustard"]
["Combine first 3 ingredients. Cover and chill."]
["sour cream", "mustard", "horseradish", "whole green peppercorns", "whole red peppercorns", "coarse salt", "beef tenderloin", "parsley", "butter", "mustard"]