Cook the potatoes in a large saucepan of lightly salted boiling water for 12–15 minutes until soft. Mash until smooth. Reserve about 4 tablespoons crème fraîche and stir the remainder into the potatoes along with the spring onions, season to taste and keep warm.
Meanwhile, stir together the butter, garlic and blue cheese, then add the walnuts and parsley and season to taste. Place in a sheet of clingfilm and roll up to form a cylinder. Twist the ends to seal and place in the freezer for 5–10 minutes to firm up.
Heat a griddle pan until smoking. Rub the oil over the steaks and season well. Cook for 2–3 minutes on each side for medium rare. Leave to rest for a minute or two, then place on serving plates with the mash and top with a slice of the flavoured butter.