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1
Make the duxelles: in a food processor, pulse the shallots, garlic, and mushrooms until the mixture is very finely minced.
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2
In a large saute pan, melt the butter over medium heat. Add the mushroom mixture to the pan and cook, stirring occasionally, until the mushrooms release their moisture and the mixture is well browned, 12-15 minutes.
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3
Season with salt and pepper. Stir in the herbs and cook for 1 minute more. Remove the mixture from the pan and cool completely.
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4
Pat the tenderloin dry with paper towels, and season all over with salt and pepper. Heat the oil in a large saute pan over medium heat.
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5
Add the tenderloin to the pan and sear until well browned on all sides, 3-4 minutes per side. Remove from pan and let cool until easy to handle, 10-15 minutes.
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6
Rub the mustard evenly over the tenderloin, then gently wrap it in the pancetta slices.
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7
Prepare a piece of plastic wrap large enough to wrap the tenderloin with. Place half of the cooled mushroom mixture in the center of the plastic wrap, and pat into an even layer. Place the tenderloin gently on top, then use your hand to apply the remaining mushroom mixture. Press the mixture firmly to coat the tenderloin all over in an even layer.
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8
Wrap the tenderloin tightly in the plastic wrap, and chill for 20-30 minutes. Towards the end of the chill time, turn the oven on to preheat to 425u00b0 F, and line a baking sheet with parchment paper.
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9
On a lightly floured surface, roll out each piece the puff pastry to 1/4-inch thick; both pieces should be slightly longer and wider than the tenderloin.
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10
Transfer one piece of pastry to the prepared baking sheet. This will be the base of the Wellington. Unwrap the beef tenderloin and gently place it in the center of the pastry. There should be about 1 inch of pastry all the way around the outside of the beef. Use a paring knife to trim away any and discard any excess dough.
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11
Egg wash the edge of the pastry, and gently drape the second piece of dough over the tenderloin. Use your fingers to gently press the dough at the edges together to seal (once again, use a paring knife to trim any excess from the top piece of dough).
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12
Use a fork with lightly floured tines to crimp all the way around the wellington. Press firmly to make sure the puff pastry pieces are sealed together.
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13
If desired, you can score the top of the pastry with a paring knife to create a decorative pattern. For a sharper look, egg wash the pastry before you score it; the score marks will be paler than the surface of the pastry and the pattern will be very clear! For a more even look, score the pastry after scoring it - the pattern will be more subtle and the pastry will be evenly golden brown. Use the tip of a sharp paring knife to barely score the surface of the dough without cutting fully through it; create any design you like!
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14
Transfer to the oven and bake until the pastry is golden brown and fully baked through, and the steak is nearly medium rare (an internal temperature of 120-125 degrees Fahrenheit), 35-40 minutes. Let rest for 10-15 minutes before slicing and serving.
