1.Process butter, paprika, anchovies, parsley, rind and half the juice until combined. Transfer butter mixture onto a piece of plastic wrap; shape into log, roll tightly. Freeze until ready to use.
2.Heat an oiled grill plate (or grill pan or barbecue). Rub beef with half the oil; season. Cook beef, over medium-high heat, for 4 minutes each side or until cooked as desired. Remove from heat, cover; rest 5 minutes.
3.Meanwhile, place watercress and capsicum in a large bowl with juice and remaining oil; toss gently to combine. Season to taste.
4.Serve beef topped with a slice of anchovy butter, and watercress salad on the side.