Crush the hazelnuts, not too finely. Warm the butter in a small saucepan. When it is a nice golden colour, add the crushed hazelnuts. Allow them to brown for 1 min. Then deglaze them with a tablespoon of water.
Set aside.
Salt the cod steaks. Keep in a cool place. Gently brown the cod steaks in a frying-pan. Turn them over and regularly baste with butter. Cook for 8 min over a medium heat, then set them aside for 5 min on a hotplate.
Cut the Jerusalem artichokes diagonally. Boil them with the Chinese artichokes in salted water, with the addition of lemon juice, a bay leaf and some thyme.
Pour the veal stock into the pan in which you cooked the cod steaks. Reduce the stock, then glaze the root vegetables in it. At the last moment, add a zest of lemon and some small sage leaves.
Serve the steaks onto heated plates. Surround the fish with the vegetables and veal stock, then drizzle the warm browned butter and crushed hazelnuts over them.
Suggestions for garnish: herbs.