1 tablespoon olive oil
1 (2-lb.) center-cut beef tenderloin, trimmed
1 1/2 pounds cremini mushrooms, washed and sliced
1 shallot, chopped
1 sprig thyme
2 tablespoons butter
1/4 cup white wine
12 thin slices prosciutto
2 tablespoons Dijon mustard
14 ounces frozen puff pastry, thawed
1 large egg, beaten
Salt and pepper
Season the tenderloin generously with salt and pepper.
Heat the olive oil in a skillet over high heat, and sear the beef on all sides until nicely browned. Set aside in the fridge.
Transfer the mushrooms, shallot and leaves from the sprig of thyme to the food processor. Pulse until finely chopped.
Heat the butter in a skillet over medium heat. Add the mushrooms and cook until brown and fairly dry, about 25 minutes.
Deglaze with wine and continue cooking until all of the liquid has cooked off. Season with salt and pepper to taste, and cool completely.
Place a sheet of plastic wrap as long as the tenderloin on the countertop.
Arrange the prosciutto slices vertically over the plastic wrap, overlapping them slightly. Spread the mushrooms over the prosciutto.
Spread the mustard evenly over the tenderloin, and place it on top of the prosciutto. Carefully roll into a log, wrapping tightly with the plastic. Chill 1 hour.
Heat the oven to 425 F.
Roll out the puff pastry on a floured work surface so it is large enough to cover the tenderloin entirely.
Remove the plastic wrap from the tenderloin, and place the meat on the puff pastry. Brush the edges of the pastry with egg wash, and roll tightly.
Trim any extra pastry, and seal the edges with a fork. Wrap in plastic and chill 20 minutes.
Remove the plastic wrap, and transfer the Wellington, seam-side down, to a foil-lined baking sheet.
Brush all over with egg wash, and bake 40 to 45 minutes, until a probe thermometer reads 120 F.
Let the meat rest 10 minutes before slicing it.
