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1
A day ahead, make the pastry.
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2
Place flour into a food processor.
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3
Cut the butter into tablespoon-size pieces and drop one by one into the flour to coat.
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4
Pulse about 5 times until butter is finely chopped.
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5
Combine the eggs, oil, 1 tablespoon water and the salt and blend with a fork.
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6
With the food processor running, pour the egg mixture in and process until dough starts to form.
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7
Stop and scrape the sides and bottom.
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8
Process just until the dough forms shiny, damp clumps, adding about 1 tablespoon more water if necessary.
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9
Turn out onto a work surface and knead lightly to make a smooth ball.
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10
Flatten into a 1 1/82-inch-thick rectangle.
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11
Cover with plastic wrap and place in a food-storage bag.
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12
Refrigerate overnight.
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13
The next day, let the dough set at room temperature for about 15 minutes.
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14
Meanwhile, sprinkle the fillet with salt and pepper.
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15
Spread the ground sausage on top, sprinkle with the truffle pieces (if using) and pat them into the sausage.
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16
Roll out pastry on a lightly floured work surface.
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17
Place the fillet, sausage side down, almost in the center of the dough.
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18
Beat 1 egg yolk with 1 teaspoon water.
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19
Fold up one long side of the pastry so that it almost covers the bottom of the fillet.
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20
Brush 1 1/82 inch of the long edge with some egg-yolk mixture.
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21
Trim the other long edge so that it just reaches over the brushed portion, and fold it up.
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22
Press lightly to seal.
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23
Flip the pastry-wrapped fillet.
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24
Seal the ends, moistening them with yolk mixture so they stick together.
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25
Place on a rimmed baking sheet.
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26
Brush pastry twice with the remaining egg-yolk mixture.
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27
Cut several slits in the top for air vents.
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28
Refrigerate while preheating the oven.
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29
Preheat oven to 400 degrees.
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30
Bake fillet until the internal temperature registers 135 degrees on an instant-read thermometer, about 35 minutes for medium-rare.
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31
If the pastry browns too quickly, cover loosely with a piece of parchment paper.
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32
Let fillet cool for 15 minutes before cutting it into thick slices.
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33
Serve warm.
