1 lb. assorted mushrooms, cleaned and thinly sliced
3 Tablespoons unsalted butter
½ cup minced onion
½ teaspoon salt plus additional to season steaks
¼ teaspoon black pepper plus additional to season steaks
4 - 7-8 oz. beef tenderloin steaks
2 Tablespoons olive oil
2 Tablespoons minced parsley
1 - 5.2 oz package garlic and herb Boursin cheese
2 sheets frozen puff pastry, thawed but still cold
1 egg, beaten
In a large sauté pan over high heat, cook the mushrooms in 2 Tablespoons of butter until they are lightly browned and cooked through. Just before removing from the heat, add the salt and pepper and cook for an additional minute. Place cooked mushrooms in a bowl. In the same sauté pan, cook the onions in the remaining 1 Tablespoon of butter over moderate heat until the onions are translucent. Add the cooked onions to the mushrooms. Set aside to cool.
Season the beef tenderloins with the additional salt and pepper. In particular, make sure you season with enough black pepper. Place a large sauté pan over high heat and let it get hot before adding the olive oil. Once you see that the olive oil is starting to smoke, place the steaks in the pan. Brown the steaks on all sides, but don't cook for more than 8 minutes. The steaks will finish cooking in the oven. When they are browned, remove them from the pan and place them in the refrigerator. It's important that the steaks are cold before wrapping in the puff pastry.
While the steaks are chilling. Add the parsley and Boursin cheese to the mushroom mixture. Stir together until well combined and place in the refrigerator to completely chill. Once the steaks and Boursin mixture are completely cold, you can proceed with the puff pastry. Place one sheet of puff pastry on a floured surface. Roll the sheet only in the lengthwise direction to make a 12 ? 14 inch rectangle. Cut the rectangle in half widthwise. Repeat with the other sheet of puff pastry. Divide the Boursin mixture equally between the four pieces of puff pastry. The mixture should be in the center of each piece and place the steak on top of this. Make sure that the steak completely covers the Boursin.
Brush the dough around the steak with beaten egg. Draw up the sides of the puff pastry toward the top of the steak, the dough should overlap slightly. You can trim off any excess dough, but make sure that all the seams are completely sealed so that the filling won't leak out. Place the wrapped steaks on a rimmed baking sheet, seam side down. Wrap in plastic wrap and refrigerate until 30 minutes before you are ready to bake. Remove from the refrigerator so that the steaks come to room temperature before baking.
Preheat the oven to 400?F. Brush the tops of the pastry with the remaining beaten egg. Place the baking sheet on a rack positioned in the center of the oven and bake for 25 minutes. After 25 minutes, insert an instant read thermometer into the top of the pastry. If it reads 125° F, remove from oven. Let the steaks rest for 4-5 minutes before serving. This temperature should yield a medium rare-medium steak. If the steak is not at this temperature, continue cooking for a few more minutes. Serves 4.