To make the triple-cooked chips:
1 Place the cut chips into a bowl under running water for 5 minutes to wash the starch off.
2 Place 2kg cold tap water in a large saucepan and add the potatoes. Place the pan over a medium heat and simmer until the chips are almost falling apart (approximately 20–30 minutes, depending on the potato).
3 Carefully remove the cooked chips and place them on a cooling rack to dry out. Then place in the freezer for at least 1 hour to remove more moisture.
4 Heat a deep-fat fryer or a deep pan no more than half filled with oil (to a depth of around 10cm) to 130ºC.
5 Fry the chips in small batches until a light crust forms (approximately 5 minutes), remove from the oil and drain on kitchen paper.
6 Put the potatoes on a cooling rack and place in the freezer for at least 1 hour. (At this stage, if you don’t want to cook and serve immediately, the chips can be kept in the fridge for 3 days.)
7 Heat the oil in the deep-fat fryer or deep pan to 180ºC and fry the chips until golden (approximately 7 minutes).
8 Drain and sprinkle with sea salt.
To make the bone marrow sauce:
1 To start the bone marrow sauce, pour the beef stock into a large saucepan and place over a high heat. Bring to the boil and allow the liquid to reduce by three-quarters until 500g remains (approximately 25 minutes).
2 In the meantime, melt the butter in a medium saucepan and cook the shallots for approximately 7–10 minutes, until they are light brown in colour. Add the wine and allow to reduce by three-quarters.
3 Remove the shallot pan from the heat and add the Dijon mustard. Stir thoroughly before adding the reduced beef stock.
4 Add the sherry vinegar and lemon juice and whisk until all the ingredients are fully incorporated.
5 Gently heat the sauce and when hot, add the diced bone marrow and remove the pan from the heat. The bone marrow should soften but not melt. Finely chop the herbs and stir them into the sauce. Season with salt and freshly ground pepper and pour into a warm jug.
To cook the steaks:
1 Heat a griddle pan until very hot, add oil and cook the steaks until done to your liking, flipping every 15 to 20 seconds.
2 Place the steaks on a cooling rack on top of a tray and allow to rest for 5 minutes.