Remove the hanger steak from the fridge an hour before you are ready to start. Preheat the oven to 80°C (176°C) (top/bottom heat). Fry the meat in the butter on both sides, then place in the middle of the oven to cook for a further 45 minutes.
For a medium steak, the core temperature should be 55°C (131°F).
For the red onion jus: peel the shallots. Caramelise the sugar in a pan. Add the raspberry vinegar, red wine and veal stock and continue to cook until the caramel is mixed in. Add the shallots and then cover and leave to simmer gently for 20 minutes.
For the celery and potato mash: peel the potatoes and celeriac, chop into small pieces and boil until soft in a covered pan of water. Drain off the liquid, then add the mixture of warm milk and cream and the cold butter to the vegetables and mash until smooth. Season to taste with the nutmeg.
For the green beans: wash the beans, then pat dry and remove the tips. Blanch until they are al dente and then transfer them into an ice bath. Leave the beans to drain. Cut the slices of ham in half and wrap them around the beans to form tiny parcels. Fry them in the butter until they are crispy all over.
Sprinkle the veal hanger steak with the fleur de sel and serve on warm plates alongside the celery and potato mash and green beans. Serve the onion jus separately.
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