1. Place the butter and marrowbone into a bowl and beat well with a wooden spoon
2. Add the capers, egg yolk, chopped chives to the mixture and a pinch of salt and mix well until incorporated
3. Spread the mixture between 2 sheets of baking paper, about 1cm thick and then place on a tray and in to the freezer to set
4. Season the fillets well with salt and pepper
5. In a hot pan add a little oil and sear the fillets on all sides until nicely caramelized. Remove from the pan and place on to a tray
6. Use a cutter roughly the same size as the fillets to cut our the marrowbone crust
7. Place on disc of the crust on to each fillet and put in to the oven for about 10-12 minutes (for medium rare)
8. Remove once cooked and place onto a wire  rack to rest for 4-5 minutes

Bourguignon Sauce

1. In a hot pan add a good splash of olive oil and then add onion, pancetta, mushroom and cook for 2-3 minutes
2. Add a good splash of verjus and cook for a further 1-2 minutes until the verjus coats the mix
3. Add the red wine sauce and bring to the boil and check the consistency then remove from the heat. Add the parsley, mix well and add a little salt if  needed

To plate

1. In the middle of a plate put a good spoon of bourguignon sauce, drizzle with a little oil.
2. Cut the side of the fillet and place on to the sauce
3. Put a good spoon of mushroom puree on and serve