The sirloin should be about 1 inch thick, so if it's too thick, pound to flatten.
Melt 2 tablespoons of butter and coat over the surface of the meat. Season with black pepper.
Meanwhile, melt remaining 1/2 cup of butter in a sauce pan. Add parsley, lemon juice and season with salt and pepper. Pour into a small container and refrigerate.
Place the Sirloin on hot grill and sear on each side for about 4 minutes. Reduce heat and continue grilling until it reaches the desired doneness (medium rare is perfect).
Remove from grill and slice into 1/2 inch thick slices.
Serve garnished with watercress and butter mixture (called Maitre d'hotel butter).