- On a work surface, generously season the roast with salt and pepper.
- In a non-stick skillet over high heat, brown the roast in the oil on all sides. Let cool on a wire rack. Transfer to a plate and refrigerate, uncovered, for 1 hour. Clean out the skillet.
- In a food processor, coarsely chop the shallots and garlic. Add the mushrooms and blend until the mixture resembles the texture of coarse breadcrumbs.
- In the same skillet over high heat, cook the mushroom mixture and thyme in the butter for 5 minutes or until any mushroom liquid has evaporated. Add the wine and let reduce until dry. Continue cooking until the mixture is lightly golden. Stir in the breadcrumbs. Set aside on a plate to cool, then refrigerate.
- In a bowl, beat the eggs and milk with a fork. Set aside.
- On a floured work surface, roll out the puff pastry to get a 20 x 11-inch (51 x 28 cm) rectangle. Starting at one of the short ends of the puff pastry, arrange the mushroom duxelles in the shape of a rectangle roughly the same size as the roast. Place the pâté slices down the middle of the rectangle of duxelles.
- Using a pastry brush, cover the roast in the mustard. Place the roast over the pâté slices. Wash the pastry brush.
- Brush the puff pastry with the egg wash. Fold the bottom edge of the dough over the roast. Tightly roll the roast to completely cover in dough. Once the roast is completely covered in puff pastry, remove any excess dough. The layer of pâté should now be on top of the roast and the seam of the dough should be underneath it. Seal the ends.
- Tightly roll the Wellington in three layers of plastic wrap. Refrigerate for 1 hour.
- With the rack in the middle position, preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
- Remove the plastic wrap from the Wellington and place on the baking sheet, seam-side down. With the tip of a sharp knife, make thin, shallow incisions along the top of the dough, keeping them close together and not going all the way through. Brush all over with egg wash.
- Bake for 10 minutes. Reduce the oven temperature to 400°F (200°C) and bake for another 25 minutes or until a thermometer inserted in the centre of the meat reads 104°F (40°C).
- Meanwhile, in a small pot over medium heat, soften the shallots in 2 tbsp of the butter. Deglaze with the wine and let reduce until almost dry. Add the stock. Bring to a boil over high heat and let reduce to 1 cup (250 ml). Season with salt and pepper. Strain through a sieve, if desired. Keep warm. When ready to serve, whisk in the remaining butter.
- Using a large spatula, transfer the Wellington to a cutting board. Let rest until a thermometer inserted in the centre reads 118°F (48°C), about 10 minutes. Cut into 6 slices. Serve with the sauce and, if desired, sautéed spinach and mashed potatoes.