Mix together mustard, mayonnaise, Worcestershire and pepper. Chill.
Preheat oven to 350 degrees F. Trim fat to 1/8-inch on sirloin, coat with oil and dredge on all sides with seasoning. Place sirloin in a well-heated (hot) iron skillet. Add 2 to 3 pats of butter. Cook 5 minutes on each side, adding butter as needed. Transfer to oven and roast until meat reaches internal temperature of 114 degrees F. Remove and refrigerate overnight.
Slice sirloin into 12 even slices and arrange on chilled salad plate. Serve with a 2-ounce pool of mustard sauce and garnish with red onion and capers.