Preheat the oven to 180C/350F. To make the Cashel blue butter, place the butter in a bowl with the Cashel blue and parsley. Season with pepper and mash with a fork until well mixed. Spoon on to a small square of parchment paper and shape into a cylinder, about 2.5cm and twist the ends. Chill for at least 1 hour to firm up.
Place the two tablespoons of the olive oil in a roasting tin with the potatoes, swede, carrots and parsnips and toss well to combine. Season with a pinch of salt and plenty of pepper and roast for 30 minutes or until the vegetables are almost but not quite tender, tossing occasionally. Add the leek to the roasting vegetable mixture, tossing to ensure that they are evenly combined. Roast for another 15-20 minutes or until all of the vegetables are completely tender and just beginning to caramelise.
Meanwhile, heat a large griddle or non-stick frying pan over a high heat. Brush the steaks lightly with oil and then season with pepper. Add to the heated pan and cook for 2 minutes on each side, then reduce the heat and cook the steak for 5-10 minutes, turning once, depending on how rare you like them. Transfer the steaks to warmed 25g butter (at room temperature).
Plate and season with salt. Unwrap the Cashel blue butter and cut into four pieces, then place one on each steak and set aside in a warm place to rest. Add the roasted root vegetables to serve.