Begin by marinating the beef cheeks. Mix together the wine, herbs, garlic and vegetables in a large dish and add the beef cheeks, ensuring they are completely covered in the liquid. Cover with clingfilm and leave to marinate for 24 hours.
The next day strain the liquid, reserving the wine, and separate the cheeks from the vegetables. Heat a little olive oil in a large pan over a high heat, then pat the beef cheeks dry with kitchen paper and add to the hot pan. Sear the cheeks evenly on all sides until golden brown and set aside.
Add the reserved vegetables to the pan and caramelise in the beef juices for a few minutes. Once golden, add the reserved wine to the pan and bring to a simmer. Reduce until the liquid has a syrup-like consistency, then return the beef cheeks to the pan and add the stock.
Bring the liquid to the boil, using a spoon to skim off any scum which appears on the surface. Reduce the heat to a gentle simmer, then cover the pan with a paper cartouche and cook for 4 hours until the cheeks are tender. Set aside and allow to cool.
Once cooled to room temperature, remove the cheeks from the pan and strain the cooking liquid through a fine sieve into a clean pan. Place this on a medium heat and cook until reduced to a sauce consistency.
Finely shred the beef cheeks with a pair of forks and mix through the reduced sauce until fully incorporated. Season with a little salt to taste, then spread the mixture out across a sheet of baking paper laid out over a large chopping board. Cover with another sheet of baking paper and roll out to an even thickness of 1cm.
Place another chopping board on top and transfer the beef cheeks to the fridge, weighted down if required. Place the bone marrow in a clean bowl of cold water to purge them of blood, leaving both the bone marrow and beef cheeks overnight to soak and press respectively.
To make the celeriac purée, add the milk and cream to a pan and place over a medium heat. Bring to the boil and grate in the fresh horseradish, adding 1 tbsp of the horseradish cream. Return to the boil, then cover the pan and set aside to infuse until cool.
Add the celeriac to the infused cream and return the pan to the heat. Bring to the boil, then reduce the heat and simmer until soft. Pass through a sieve and blitz the celeriac in a blender with the butter and a little of the cooking liquid, adding more if required to form a smooth purée. Mix through the remaining horseradish cream and season to taste, then transfer to a piping bag and keep warm.
Preheat the oven to 180°C/gas mark 4. Place an ovenproof frying pan over a high heat. Season the beef fillet with salt and pepper then sear in the pan until well coloured all over. Transfer to the oven to cook for 10–15 minutes.
Meanwhile, cook the shallots. Heat the butter in a pan until foaming and add the whole peeled shallots. Cook until golden brown all over, then strain off any remaining butter from the pan. Add the cassis and reduce the liquid to a thick syrup, then add the chicken stock and simmer until the shallots are tender. Set aside, keeping warm until required.
For the carrots, melt the butter in a separate pan over a medium heat. Once the butter begins foaming add the peeled carrots, thyme and garlic and fry until tender and slightly charred. Season with a little salt and set aside, keeping warm until ready to serve.
Remove the beef fillet from the oven and leave to rest, covered, for 10 minutes. Meanwhile, drain the bone marrow and remove the marrow from the bones. Heat the chicken stock in a small pan until gently simmering, then slice the bone marrow into 5mm discs and poach gently for 3 minutes.
Cut the pressed beef cheeks into even rectangles and place in the centre of each serving plate, warming through under a hot grill. Meanwhile, carve the beef fillet into 8 slices.
To serve, arrange slices of beef fillet on top of the pressed beef cheeks and pipe dots of the celeriac purée across the length of the beef cheek. Add the warm shallots and carrots to the plate and top with slices of bone marrow, garnishing with watercress leaves.