A beef tenderloin coated in mustard, wrapped in a savory mushroom duxelles and a layer of prosciutto, then encased in puff pastry and baked until the pastry is golden and the beef is rosy pink inside - searing the beef first and cooking the duxelles until completely dry are what keep the pastry from turning soggy underneath.
1.Season the beef tenderloin generously with salt and pepper, then sear in the vegetable oil in a very hot pan for 1-2 minutes per side until browned all over but still raw inside. Let cool, then brush all over with the dijon mustard.
2.Pulse the mushrooms, shallots, garlic, and thyme in a food processor until finely minced. Cook this mixture in the same pan over medium heat, stirring often, for 10-15 minutes until all the liquid has evaporated and it forms a dry, dense paste (the duxelles) - any remaining moisture here is what makes the pastry soggy later, so cook it until it's genuinely dry, not just soft.
3.Lay a large sheet of plastic wrap on the counter and arrange the prosciutto slices in a slightly overlapping layer. Spread the cooled duxelles evenly over the prosciutto.
4.Place the mustard-coated beef in the center and use the plastic wrap to roll the prosciutto and duxelles tightly around it, twisting the ends to seal. Chill in the refrigerator for 30 minutes to set the shape.
5.Preheat the oven to 425°F. Roll out the puff pastry on a floured surface, unwrap the beef, and place it in the center. Wrap the pastry around the beef, sealing the seams with a little egg wash, and trim any excess.