2 Tbs. olive oil
1 medium bulb fennel, cored and roughly chopped (2 cups)
1 medium apple, peeled, cored, and diced (1 cup)
1/2 tsp. red pepper flakes
Kosher salt and freshly ground black pepper
2 Tbs. Maille Dijon mustard; more for serving
1 pork tenderloin (about 1 lb.)
1 (14-oz.) sheet frozen puff pastry, thawed
4 oz. thinly sliced prosciutto
1 large egg, beaten
Step 1: In a large skillet, heat the olive oil over medium-high heat until shimmering. Add the fennel, apple, and red pepper flakes and season lightly with salt and pepper.
Step 2: Cook, stirring occasionally with a wooden spoon, until deeply golden, about 10 minutes. Remove from the heat, stir in the mustard, and let cool completely.
Step 3: Line a rimmed baking sheet with parchment paper. On a lightly floured surface, roll the puff pastry to a rectangle roughly 15 x 12 in., with the long end facing you.
Step 4: Spread the fennel mixture in an even layer over the pastry, leaving a 1/2-inch border. Brush the border with the beaten egg. Shingle the prosciutto over the fennel to cover so that it is slightly overlapping, about 4 slices per row.
Step 5: Season the pork tenderloin lightly with salt and pepper, place it along the bottom edge of the pastry, and fold 1 to 2 inches of the tapered end toward the thicker end to make for a more even roast.
Step 6: Hold the roast to secure, and roll the pastry and tenderloin away from you, using a bench scraper if necessary to help facilitate rolling.
Step 7: Turn the roll seam side up. Brush the overhanging dough with egg, and then fold the overhang up toward the seam and press gently to seal.
Step 8: Place the Wellington on the prepared baking sheet, seam side down. Refrigerate the roast for at least 30 minutes.
Step 9: Position a rack in the center of the oven and heat the oven to 425°F.
Step 10: Brush with the egg, and using a sharp paring knife, cut 4 to 5 incisions along the top of the pastry to vent. Transfer to the oven and bake, rotating halfway through, until the pastry is golden and the roast registers 140°F to 145°F, in the center, about 35 minutes.
Step 11: Remove the roast from the oven and transfer to a cooling rack. Let rest for 5 minutes before serving with additional mustard on the side.