Add all of the ingredients into a pot and bring up to the boil and turn down to a simmer. Cool for 5 hours and keep an eye on the liquid as you might need to top up as you go.
After 5 hours the meat should fall apart. Turn the heat down to a low temperature, leaving all the vegetables inside. Stir it well and reduce until all the liquid is gone (the drier the mix the better the result!). Stir occasionally to stop the meat sticking to the bottom of the pot.
Once dry, place in the fridge and let this cool overnight.
Mix the grated potato with salt, egg and buttermilk. Mix the mash and flour together and whisk in the wet mix (this should have a batter texture that is similar to pouring cream). Add a splash of water in case needed.
Heat up your largest frying pan and add a teaspoon of oil, pouring in enough mix to cover the pan. Cook for 30 seconds and check if it is golden brown on the bottom. Once golden brown, flip over for another 30 seconds. If you have any boxty crepes left over, place on a plate with cling film and store in the fridge.
BEEF AND GRAVY
Season your Chateaubriand well with salt and pepper. Heat up a large frying pan add 3 table spoons of oil, searing each side of the beef until it is a golden brown.
Remove the beef out of the pan and leave to rest. Add shallots, garlic, rosemary and red wine and bring to a boil, adding in chicken stock and cook for 5 minutes. Turn down the heat and keep until you are ready to serve.
WRAPPING BEEF IN BOXTY CREPE
Cling film a flat work surface and place your boxty crepe on the cling film, ensuring there is enough to wrap around your beef with plenty of the crepe to overlap.
Spread your braised beef onto the boxty crepe then place your seared beef on the edge of the pancake, and roll wrap in cling film. Leave to rest in the fridge while you roll out your pastry.
Dust your work surface in flour and roll your pastry large enough to envelop your beef wrapped in the boxty crepe.
Egg wash the pastry and place the beef wrapped in the boxty crepe on the edge. Roll until the fold is on the bottom.
Cut a piece of grease proof paper to fit under the Wellington. Finish with more egg wash and then rest in the fridge until you are ready to cook.
COOKING THE WELLINGTON
Pre-heat the oven to 200°c. Place a heavy tray in the pre-heated oven for at least 10 minutes before adding in the Wellington, to ensure a more crispy pastry base.
Once added, leave to cool in the oven for 32 minutes and then rest for at least 10 minutes before serving.
Cut in half and cut each half again. Serve with your favourite trimmings such as glazed carrots, roast potatoes and red wine gravy.