Tightly bind the beef with string and season with salt and pepper.
Heat up a dash of olive oil in a pan and sear each side of the meat in the hot oil. Leave this to rest somewhere cool.
Peel and finely slice the mushrooms.
In a saucepan, sweat the shallots with the butter and a pinch of salt.
Add the mushrooms and the white wine and cook over a high heat, until all the liquid has evaporated.
Add the breadcrumbs and season.
Beat the egg.
Preheat the oven to 180'C.
Roll out the pastry into a rectangular shape, slightly bigger than the meat.
Brush the edges of the pastry with the beaten egg.
Spread the mushroom mixture out in the middle of the pastry, place the meat on top and fold the pastry over, covering the filling.
Brush with the egg yolk and then bake for 30 minutes. Once cooked, leave this to rest for a further 20 minutes.
Peel the celeriac and cut into 1 inch thick batons, wash the carrots and scrub them if needed.
Place the vegetables in a baking dish with the garlic, thyme and olive oil and season with salt and pepper.
Boil the balsamic down by half
Cook for 20 minutes, then turn the vegetables and drizzle with balsamic vinegar.
Cook for a further 10 minutes.
Slice the beef wellington into 6 and serve with the roasted root vegetables.
Pick the top leaves of watercress and garnish the plate with them.
«You should make sure to rest the beef well before and after cooking. An ideal temperature for the middle of the beef would by 45 degrees when you take it out of the oven for a medium rare finish, dont forget it will keep on cooking for a while!»