Cover the bone marrow in water, cover and soak in the fridge overnight. In the morning, drain and cut into 2.5cm/1in pieces.
To make the sauce, put the wine, shallots, peppercorns, herbs and garlic in a large saucepan over a high heat. Bring to the boil and simmer until the volume of the liquid has reduced by a third.
Add the stock, bring back to the boil and simmer until the volume of the liquid has reduced by half or until it coats the back of a spoon.
Place the bone marrow in a saucepan with a little water. Bring to the boil and cook for 30 seconds, then drain. Whisk the bone marrow and butter into the reduced sauce and season with salt and pepper. Keep warm.
Blanch the chips in boiling water for 5–8 minutes until softened but not cooked. Drain and spread out on a baking tray and chill in the fridge.
Preheat a deep-fat fryer to 130C (CAUTION: hot oil can be dangerous. Do not leave unattended.). Cook the chips for 5 minutes, or until cooked, but not coloured. Drain, spread out on the baking tray and leave to cool.
Turn the fryer up to180C. Cook the chips for 4–5 minutes until crisp and golden. Drain on kitchen paper and sprinkle with salt and malt vinegar.
Preheat the grill to its highest setting and cook the steaks for 4 minutes on each side or to your liking.
Serve with the sauce poured over and the chips on the side. Garnish with the watercress.