Beef Wellington

Method

1. 1Sear the beef and chill. Season the tenderloin all over with 1½ teaspoons of the fine salt and the black pepper. Heat a heavy skillet or cast-iron pan over the highest possible heat with the neutral oil until the oil just starts to smoke. Sear the beef on every surface, including both ends, for 45 to 60 seconds each, about 4 to 5 minutes in all, turning as soon as each face is deep brown; take it off then, because the inside must stay completely raw. Lift it out, snip off and discard the string, brush all over immediately with Dijon mustard, and leave to cool completely on a wire rack. Do not wrap warm beef, the steam will ruin the pastry. Refrigerate uncovered for at least 30 minutes.
2. 2Make the mushroom duxelles. Pulse the mushrooms in a food processor in two batches, 10 to 15 seconds each, until the pieces are the size of rice grains, not a purée (or chop them very finely by knife). In the same pan over medium heat, melt the butter and soften the shallots and garlic for 2 minutes. Add the mushrooms, thyme leaves and the remaining ½ teaspoon of fine salt. Cook, stirring frequently, over medium-high heat for 15 to 20 minutes: the mushrooms first release a lot of liquid, and you keep going until it has all cooked off. Add the wine and cook until completely evaporated. Continue until the mixture is dark, dry and almost paste-like; when you press a spoonful against the pan, no liquid should run. Spread on a plate to cool completely. This step is the single most important technique in the recipe: any residual moisture will make the pastry soggy.
3. 3Wrap in prosciutto and chill. Lay a large sheet of cling film on a work surface. Arrange the prosciutto slices in a slightly overlapping rectangle roughly the length of the beef. Spread the cooled duxelles evenly over the prosciutto. Place the cold, string-free beef at the bottom edge and, using the cling film, roll the prosciutto tightly around the beef, twisting the ends to form a firm, even cylinder. Refrigerate for at least 20 minutes. This cold, firm roll is essential for clean assembly.
4. 4Wrap in pastry and chill again. Roll the cold puff pastry on a lightly floured surface to a rectangle about 3 mm thick, large enough to wrap the beef with a 5 cm overlap on all sides. Unwrap the cling film from the prosciutto roll and place the roll seam-side-down at one edge of the pastry. Brush the far edges with egg wash, roll the pastry tightly around the beef, and press to seal all seams. Trim the excess from the ends, fold neatly, and press. Place seam-side-down on a baking sheet lined with baking paper. Brush all over with egg wash. Using the back of a knife, score a diagonal pattern across the top. Refrigerate uncovered for at least 20 minutes, or loosely covered for up to 24 hours.
5. 5Bake and rest. Put a heavy baking tray in the oven and preheat it to 200°C (400°F) for at least 15 minutes. Remove the Wellington from the fridge, brush again with egg wash, sprinkle flaky salt over the top, and slide it, with its baking paper, onto the hot tray. Bake for about 30 minutes, checking from minute 25: the pastry must be deeply golden (a pale Wellington is undercooked), and a probe thermometer in the center of the beef should read 46 to 48°C (115 to 118°F) for medium-rare. Remove from the oven and rest on the tray for 15 minutes without cutting; the center climbs to about 52 to 54°C (125 to 130°F) as it rests. Food-safety agencies set 63°C (145°F) plus a 3-minute rest as the safe minimum for a whole cut of beef; medium-rare is below that line, a choice many cooks make with an intact fillet. For children, pregnant women, older people and anyone with a weak immune system, take the Wellington out at 57°C and rest it to 63°C: the center will be light pink rather than red. Slice at the table with a sharp serrated knife; the first cut reveals the dramatic cross-section that defines the dish.