Beef Wellington

Steps

1. Tie the beef with kitchen twine every 1 1/2 inches so it holds an even, round shape. Rub it all over with the 1 1/2 teaspoons of salt and the pepper.
2. Heat the oil in a 12-inch skillet over medium-high heat until it shimmers. Brown the beef on all sides, including the ends, 8 to 10 minutes in all. Move it to a wire rack to cool and keep the skillet handy for the mushrooms.
3. Pulse the mushrooms in a food processor in two batches until they look like coarse crumbs, about 10 pulses per batch. Stop before they turn to paste.
4. Melt the butter in the same skillet over medium-high heat. Add the shallots, garlic and thyme and cook, stirring, until soft and fragrant, about 2 minutes. Add the mushrooms and the 1/2 teaspoon of salt. Cook, stirring often, until all their liquid has cooked away and the mixture is dry, crumbly and just starting to stick, 20 to 25 minutes. Add the sherry, if using, and stir until the pan is dry again, about 1 minute. Take your time here, because wet mushrooms make soggy pastry.
5. Spread the mushrooms on a plate and refrigerate until cool, about 20 minutes.
6. Overlap two long sheets of plastic wrap on the counter. Lay the prosciutto on them in overlapping rows to make a rectangle about 12 by 14 inches. Spread the cooled mushrooms over it in a thin, even layer.
7. Snip the twine off the beef and brush it all over with the mustard. Set it along a short edge of the rectangle. Using the plastic wrap to help, roll the prosciutto and mushrooms snugly around the beef. Twist the ends of the plastic like a candy wrapper to make a tight log, and refrigerate for 30 minutes.
8. On a lightly floured counter, overlap the two pastry sheets by about 1/2 inch and press the seam together. Roll into a rectangle about 14 by 16 inches. Unwrap the beef and set it along a 14-inch edge. Brush the edges of the pastry with egg wash.
9. Roll the pastry around the beef until the long edges overlap by about 1 inch, trimming any extra so the seam is not too thick. Press the seam closed. Fold in the ends like wrapping a gift, trim any thick folds and press to seal. Set it seam side down on a parchment-lined rimmed baking sheet and refrigerate until cold and firm, at least 30 minutes or up to 3 hours.
10. About 20 minutes before baking, heat the oven to 425°F with a rack in the lower third. Brush the pastry all over with egg wash. With the back of a paring knife, lightly score a crosshatch on top without cutting through, and sprinkle with flaky salt, if using.
11. Bake until the pastry is deep golden brown and a thermometer pushed into the center of the beef reads 120°F for medium-rare or 130°F for medium, 40 to 50 minutes. Start checking after 35 minutes. If the top browns too fast, lay a piece of foil loosely over it.
12. Slide the Wellington onto a wire rack and let it rest for 15 minutes. The center will rise about 10 degrees. Cut into 1-inch slices with a serrated knife.

Good to know

• Make ahead: cook the mushrooms up to 2 days ahead and refrigerate them. You can finish the prosciutto log up to 8 hours ahead. Add the pastry within 3 hours of baking so it stays crisp. Leftovers keep 3 to 4 days in the refrigerator.
• Alcohol-free: leave out the sherry and stir in 1 teaspoon of Worcestershire sauce at the same point.
• Swap: thin-sliced deli ham works in place of the prosciutto, and a mix of white button and shiitake mushrooms works in place of the cremini.
• The USDA recommends cooking beef roasts to 145°F, then letting them rest at least 3 minutes before carving. For beef cooked to that point, keep baking until the center reads 145°F, with foil loosely over the top so the pastry does not burn.