For anchovy and rosemary butter, melt butter in a small saucepan over medium-high heat, add anchovy, shallot, garlic and rosemary and sauté until shallot is soft (2-3 minutes). Remove from heat and stir through mustard and vinegar. Set butter aside and keep warm until required.
Preheat a char-grill over high heat, brush steak with olive oil, season to taste and cook until medium-rare (3-4 minutes each side). Place on a warmed plate, cover with foil and rest for 2 minutes, then serve with watercress sprigs and anchovy and rosemary butter.