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1
Sprinkle the tenderloin generously on all sides with salt and pepper.
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2
Heat 2 tablespoons oil in a large skillet over high heat.
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3
Sear the beef on all sides, 8 to 10 minutes.
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4
Remove to a baking sheet and cool completely, about 20 minutes.
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5
Add half of the mushrooms to a food processor and pulse until they are very finely chopped.
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6
Remove the finely chopped mushrooms to a bowl and continue with the other half of the mushrooms.
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7
Reserve.
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8
Heat the butter and remaining tablespoon of oil in a large pan over medium-high heat.
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9
Once melted and foaming, add the shallots and sweat until fully softened, about 4 minutes.
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10
Add the mushrooms to the pan and saute until they release all their moisture and then dry, about 10 minutes.
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11
Add the heavy cream and simmer until thick, about 2 minutes.
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12
Turn off the heat and stir in the breadcrumbs and parsley.
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13
Remove the mushrooms to a small bowl, cool completely and chill in the refrigerator for at least 1 hour.
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14
The mushrooms should now look like a paste.
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15
Combine the Dijon and horseradish in a small bowl.
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16
Once the meat is cool to the touch, snip off the butcher's twine and discard, and then spread the mustard and horseradish mixture all over the beef.
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17
Place in the refrigerator to chill for 1 hour.
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18
Adjust a rack to the center of the oven and preheat to 400 degrees F. Line a baking sheet with parchment paper.
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19
Crack the egg into a small bowl and beat until smooth.
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20
Dust the work area lightly with flour.
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21
Roll out the puff pastry with a rolling pin so it's 4 inches longer than the tenderloin and 6 to 7 inches wider.
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22
Brush a 3-inch edge of the pastry with the beaten egg.
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23
Spread and pat the mushroom mixture all over the top and sides of the beef.
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24
Lay the beef mushroom-side down in the center of the pastry and pat the remaining mushrooms onto the exposed side.
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25
Tuck the 2 ends of the pastry into the beef and then fold up the other sides.
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26
Place the beef seam-side down on the baking sheet.
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27
Brush all over with the egg wash. Cut 3 slits on the top of the roast and sprinkle lightly with flaky sea salt.
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28
Roast until a thermometer inserted into the beef reads 120 degrees F, about 40 minutes.
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29
Remove from the oven and rest for 20 minutes before slicing into 1-inch-thick pieces.
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30
The temperature will rise to 125 degrees F, leaving you with a nice ruby red medium rare.
