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1
Place the porcinis in a small heatproof bowl and pour in the boiling water.
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2
Let sit until completely softened, about 30 minutes.
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3
Using a fork, transfer the porcinis to a cutting board (be careful not to disturb the gritty sediment at the bottom of the bowl).
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4
Finely chop the mushrooms and set them aside.
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5
Slowly pour the soaking liquid into a small bowl, leaving the sediment behind; set the liquid aside and discard the sediment.
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6
Season the beef all over with salt and pepper.
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7
Melt 1 1/2 tablespoons of the butter in a large frying pan over medium-high heat until foaming.
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8
Place the roast in the pan and brown it all over, taking care not to burn the butter, about 5 to 6 minutes total.
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9
Transfer the roast to a plate to cool.
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10
Melt the remaining 1 1/2 tablespoons butter in a separate medium frying pan over medium heat.
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11
Add the shallots and cook for 1 minute.
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12
Add the reserved porcinis, the garlic, the reserved porcini liquid, and the cremini mushrooms and stir to combine.
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13
Increase the heat to medium high and cook, stirring occasionally, until the mushroom mixture has released most of its moisture and appears dry, about 10 minutes.
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14
Season with salt and pepper, add the thyme, and stir to combine.
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15
Set aside to cool to room temperature, about 20 minutes.
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16
When the mushroom mixture has cooled, place the pate in a medium bowl and beat with a rubber spatula until smooth.
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17
Add the mushroom mixture and stir to evenly combine.
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18
Taste and season with salt and pepper as needed.
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19
Using a long metal spatula, spread half of the mushroom mixture evenly over one side of the beef roast.
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20
Lay a sheet of plastic wrap, slightly larger than the roast, horizontally on the work surface.
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21
Vertically arrange half of the prosciutto, one piece slightly overlapping the next, along the length of the plastic wrap (it should be about the same size as the roast).
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22
Place the roast mushroom-side down on the prosciutto.
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23
Spread the remaining mushroom mixture over the roast.
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24
Lay the rest of the prosciutto slices, in the same manner as before, over the mushroom mixture.
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25
Fold up the edges of the plastic wrap and use another piece of plastic to enclose the entire roast; refrigerate.
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26
Heat the oven to 425 degrees F and arrange a rack in the middle.
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27
Cut off about one-third of the puff pastry from the sheet and roll it out on a lightly floured surface into a rectangle 1/8 inch thick and at least 1 inch larger than the base of the roast.
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28
Transfer to a baking sheet.
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29
Prick well with a fork all over and bake until brown and crisp, about 12 to 15 minutes.
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30
Transfer the pastry to a cutting board to cool, then trim it to the size of the roast.
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31
Return the pastry to the baking sheet; set aside.
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32
Remove the roast from the refrigerator.
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33
Remove and discard the plastic wrap.
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34
Brush the roast all over with the beaten egg; reserve the remaining egg.
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35
Roll out the remaining puff pastry on a lightly floured surface into a rectangle about 12 by 14 inches.
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36
Place the roast on top of the cooked pastry base.
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37
Lay the rolled-out pastry over the roast.
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38
Use a spatula to lift up the pastry base, then tuck the sides of the uncooked pastry underneath the cooked pastry to seal the roast.
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39
Brush the pastry all over with the reserved egg.
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40
Roast for 20 minutes.
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41
Lower the oven temperature to 400 degrees F and continue to roast for 25 minutes more for rare to medium-rare beef, 30 to 35 minutes for medium.
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42
Remove the Wellington from the oven and let it stand for about 10 minutes before slicing.
