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 40 minutes. Once cooked, leave this to rest for a further 20 minutes.
Cut the fennel bulbs into quarters and remove the core.
Place them in a baking dish with the garlic, thyme and olive oil and season with salt and pepper.
Cook for 20 minutes, then turn the bulbs and drizzle with balsamic vinegar.
Cook for a further 10 minutes.
Slice the beef-en-croute and serve with the fennel.
«Leave to rest when cooked to let the flavours develop.»