Fry the meat well on all sides to seal all the spores well.
Season the meat with salt, pepper and grease well on all sides with mustard.
In a food processor, crush the pistachio and mix it with caramelized onions.
Spread the puff pastry dough just enough that will allow you to wrap the meat on all sides.
Grease the caramelized onion mixture on the dough.
Put the meat in the middle and cover well on all sides of the dough, try to avoid too thick edges.
Line a baking tray with baking paper then place the meat wrapped in the dough on it with the side where the dough sticks down.
Grease well with a beaten egg, then make a few scratches with a fork for a more attractive look at the end.
Preheat the oven and bake the beef wellington at 350F for about 40 minutes.
Let the meat to rest for 20 minutes, then you can cut and serve.
