Preheat the oven to 392°F (200 °C). Prepare a flat tray covered with baking paper.
Scrub the mushrooms with a moist paper towel and peel them. Put them in a blender with the salt, black pepper and thyme, blend to a paste for about 30-60 sec. Transfer to a medium-sized pan with about 1 tbsp oil, distribute along the bottom like a pancake and cook on a moderate stove, while stirring often.
The mushrooms will initially release their own water, which needs to evaporate, the paste will shrink to 1/2 its original size, thicken and become nearly dry. Remove from the heat and leave it aside.
Season the piece of meat (or 2 pieces) with a little salt and black pepper on both sides. Put in a pan with highly heated oil to crisp on both sides, including their edges. Take out and leave to rest about 5 min. Then smear generously with mustard on both sides.
Arrange the prosciutto onto a rolled out sheet of puff pastry, overlapping them slightly, put 1/2 tbsp of the mushroom mixture on top, spread it out and place the smeared pieces of meat on top. Smear with mushroom paste again and cover with the prosciutto.
Smear the sides of the puff pastry with a brush, so that they stick afterward, and using the foil roll it up into a tight roll around the piece of meat- fold the right side on top, then cover with the left.
Press so that they stick to each other. Wrap the foil around the sides tightly like a bonbon and place the roll in the fridge for 15 min. to harden. Then unwrap from the foil, place into the tray with baking paper, smear generously with the egg and make slight incisions without cutting the puff pastry deeply.
Bake at 392°F (200 °C) about 30 min.
Garnish - your choice.
Source: Gordon Ramsay
Notes: This was my attempt to make the famous Wellington roll which Americans and residents of English-speaking countries prepare for Christmas. Usually it's made with veal tenderloin but I used pork instead. The result was still spectacular. It's easy, elegant, incredibly tasty, doesn't take too long.
When making the mushroom paste, you can optionally add about 5.5 oz (150 g) already prepared chestnuts to the blender.
You can use 1 whole piece of meat or 2 separate ones.
You need a fully dried prosciutto because if there's any moisture in it it won't be good for the puff pastry.
Use minimum amounts of salt because the prosciutto is plenty salty.
Pinch closed the edges of the puff pastry very well.
You can prepare it the night before, leave in the fridge and bake the next day. According to Gordon Ramsay that's the better way to do it because that way the puff pastry will harden enough and the roll will have a better form.
After smearing well with egg and making cuts you can pour on a little kosher salt.
After baking, be sure to leave the roll to rest 10 min. and only then cut it into slices.