Place potato in a large saucepan of cold salted water, bring to the boil over high heat, cook until tender (10-15 minutes). Drain, return to pan with butter, mash well then add cream. Season to taste, keep warm.
Meanwhile, combine steak, olive oil, garlic, rosemary and half the wine in a non-reactive dish, season to taste, turn to coat, set aside.
For rosemary butter, combine ingredients in a bowl, season to taste, set aside.
Heat a large frying pan over high heat. Drain beef from marinade, add to pan and cook, turning once, until medium-rare (3-4 minutes each side). Remove from pan, cover with foil and rest (5 minutes). Deglaze pan with remaining wine, add stock, simmer until reduced to a glaze (3-4 minutes). Serve steak with sauce, mash, rosemary butter and beans.