How to Make It
Preheat the oven to 375°. Spread one-fourth of the celery root slices in a shallow 1-quart gratin dish. Season with salt, pepper and nutmeg. Repeat with the remaining celery root slices to create 4 layers; season between each layer. Pour the cream over the celery root and dot with the 1 tablespoon of butter pieces. Cover with foil and bake for 30 minutes. Remove the foil and bake for about 30 minutes longer, until browned on top and the celery root is tender when pierced with a knife. Let stand for 15 minutes. Leave the oven on.
In a large ovenproof skillet, melt 1 tablespoon of the butter in the olive oil. Season the steaks with salt and pepper and cook over moderately high heat until a brown crust forms on the bottom, about 3 minutes. Turn the steaks, transfer the skillet to the oven and cook for about 12 minutes for medium-rare. Transfer the steaks to plates.
Add the veal demiglace to the skillet and bring to a boil. Remove the skillet from the heat and whisk in the remaining 3 tablespoons of butter, 1 tablespoon at a time. Season the sauce with salt and pepper. Spoon the sauce over and around the steaks. Garnish each steak with a sprig of rosemary and serve hot with the celery root gratin.