21st-Century Beef Wellington

FOR THE FILLING:

Have a large bowl of ice water at hand.

Bring a large pot of water to a boil over medium-high heat. Add a generous amount of salt, then the spinach; cook for 30 seconds; add the cilantro and parsley and cook for 10 seconds. Pour into a colander to drain, then quickly transfer to the bowl of ice water to stop the cooking. Drain in a colander, then remove the mixture in small handfuls, squeezing to eliminate as much water as possible.

Pulse the walnuts in a food processor just until finely ground. Add the spinach mixture along with the garlic, bread crumbs, honey, egg whites, the 1 1/2 teaspoons of salt, the ground cumin and coriander and the pepper; pulse just until smooth, with the consistency of a thick pesto. The filling can be made up to 2 days in advance, covered and refrigerated.

FOR THE PASTRY:

Lightly flour a work surface.

Combine the 3 1/4 cups of flour, the butter and salt in a bowl and use your fingertips or a pastry blender (or use a food processor) to blend until the mixture resembles coarse meal, with some pea-size lumps. Add the sour cream and stir with a fork (or pulse in the processor) just until incorporated. Drizzle evenly with 4 tablespoons of the ice water and gently stir (or pulse) until incorporated. Squeeze a small handful of dough; if it doesn't hold together, add more ice water 1 tablespoon at a time, stirring (or pulsing) just until incorporated.

Turn the mixture out onto the floured work surface and divide it into 4 equal portions. Use the heel of your hand to smear each portion once or twice in a forward motion to help distribute the fat. Gather the portions of dough together and form them into a single 4-by-5-inch rectangle. Wrap in plastic wrap and refrigerate until firm, for at least 2 hours and up to 1 day.

FOR THE BEEF AND ASSEMBLY:

Pat the tenderloin dry. Cut it in half crosswise and sprinkle it all over with the salt and pepper.

Heat the oil in a large, heavy skillet over medium-high heat until nearly smoking. Working with one piece at a time as needed, add the beef and sear, turning it with tongs, until browned on all sides, about 5 minutes total. Transfer to a platter. Remove any string.

Lightly whisk the egg and water together to form an egg wash.

Lightly flour a work surface and roll out the dough to a 15-by-19-inch rectangle; it should be slightly less than 1/4-inch thick. If you plan to decorate the pastry crust, cut a 1-inch strip of dough from a short end and refrigerate it. Place the dough rectangle lengthwise on a large baking sheet, letting the excess hang over the ends. Spread a generous 1/4 of the filling down the middle of the dough, forming a 3-by-16-inch strip. Lightly pat the beef pieces dry, arrange them end to end on the filling, and spread them with the remaining filling mixture to cover completely. Brush the border of the dough with the egg wash, then fold up the long sides of the dough to enclose the beef tightly and completely, reserving any excess, and press the seam to seal. Fold up the short ends and seal the edges, cutting off any excess and reserving it. Refrigerate any reserved dough pieces.

Line a large baking sheet (preferably rimmed) with parchment paper and invert it over the beef. Hold both baking sheets together and invert the beef so the pastry is now seam side down. Brush the dough evenly with the egg wash. Cut out decorative shapes from the reserved, chilled pastry dough, if desired, and arrange them on the dough, pressing gently to adhere. Brush the decorations with the egg wash. Use the tip of a small paring knife or the point of a skewer to make small steam holes or vents every 3 inches across the top of the pastry. Loosely cover the assembled Wellington and refrigerate for at least 1 hour and up to 6 hours.

Preheat the oven to 400 degrees. Remove the Wellington from the refrigerator and bake (middle rack) for 45 to 55 minutes, rotating the sheet front to back after about 25 minutes, until the pastry is golden brown and an instant-read thermometer inserted into the center of the meat registers 115 degrees. Transfer the baking sheet to a cooling rack and let the beef Wellington rest for 25 minutes before slicing. (It will continue to cook, reaching 125 to 130 degrees for medium-rare.)

NOTE: To toast the walnuts, spread in a single layer on a baking sheet and bake in a preheated 350-degree oven, stirring once or twice, for about 8 to 12 minutes or until they darken and smell toasty. Check frequently; nuts can burn quickly.