For the gnocchi, mix all of the gnocchi ingredients together until well combined as a dough, using your hands. (Do not overwork the mixture.)
Turn out the dough onto a lightly floured work surface and roll out to a 0.5cm/½in thickness. Cut the dough into 1.5cm x 3cm/½in x 1in pieces.
Bring a pan of salted water to the boil. Carefully lower the gnocchi into the pan, in batches, then blanch for 1-2 minutes.
Drain well and place onto a lightly oiled baking tray. Chill in the fridge until needed.
For the white sauce, preheat the grill to its highest setting.
Heat the butter in a saucepan over a low to medium heat until melted. Whisk in the flour until smooth and well combined. (This is called a roux.) Remove from the heat and set aside to cool.
Bring the milk to the boil in a separate pan. Add the remaining white sauce ingredients except the chervil, then remove the pan from the heat and set aside for 20 minutes to infuse.
When the milk has infused, pass the it through a fine sieve into a clean saucepan (discard the sieve contents). Whisk the cooled roux into the hot milk and continue to heat until the mixture is boiling and has thickened. Continue to boil for 1-2 minutes, then remove the pan from the heat.
Meanwhile, for the bone marrow crust, heat the bone marrow and butter in a saucepan over a medium heat until melted and well combined. Remove from the heat, then stir in the brioche crumbs. Set aside.
For the mushrooms, heat the oil in a frying pan over a medium heat. Add the mushrooms and fry for 1-2 minutes, or until softened. Season, to taste, with salt and freshly ground black pepper.
Transfer the gnocchi to an ovenproof dish and pour over the white sauce. Stir in the cooked mushrooms and chervil until well combined. Spoon over the bone marrow crust.
Place the ovenproof dish onto a baking tray and grill until golden-brown and bubbling.
Meanwhile, for the cöte de boeuf, preheat the oven to 200C/400F/Gas 6.
Rub the beef all over with the oil, then season on both sides, to taste, with salt and freshly ground black pepper.
Heat a griddle pan over a high heat until very hot. When the pan is hot, add the beef and fry on both sides for one minute, or until golden-brown chargrill marks appear on both sides of the beef.
Transfer the griddled beef to a roasting tray and roast in the oven for 12-14 minutes (for rare), or until cooked to your liking. Remove from the roasting tray and set aside on a warm plate to rest for at least five minutes.
For the cavolo nero, blanch the cavolo nero leaves for 20-30 seconds in a pan of salted, boiling water. Drain well.
Heat the butter in a frying pan over a medium heat. When the butter is foaming, add the shallots and blanched cavolo nero and fry for 1-2 minutes, or until the cavolo nero is wilted and the shallots have softened.
To serve, carve the cöte de boeuf into thick slices and divide them equally among four serving plates. Spoon the bone marrow gratin and cavolo nero alongside.