Chop the mushrooms and dice the peeled celeriac into small cubes.
Thinly slice the shallot and garlic.
Heat 1 Tbsp of the oil in a pan and gently sauté the shallot over low heat for about 5 minutes, until soft.
Add the garlic, herbs, celeriac and mushrooms. Stir for a minute, then pour in the stock.
Cover and cook for 10–15 minutes, until the celeriac is tender. Add a little more stock, or a splash of single cream, if the sauce becomes too dry.
Meanwhile, rub the steaks with the remaining oil and season generously with salt and pepper.
Heat a griddle pan until very hot and cook the steaks for 3–5 minutes on each side, depending on thickness and how well done you like them.
If using blue cheese, place a slice on each steak during the last minute of cooking and cover the pan briefly to let it melt.
Transfer the steaks to a plate and cover loosely. Allow them to rest for 5 minutes.
Spoon the celeriac and mushroom sauce onto plates, place the steaks on top and garnish with chopped parsley.
Serve immediately.