Step 1

Heat the olive oil in a large skillet over medium-high heat. Pat the beef dry with a paper towel and season all sides with salt and pepper. Quickly sear the beef on 2 sides only until deep golden brown, about 4 minutes total; do not overcook. Transfer to a plate to cool.

Step 2

Add the mushrooms and cook until beginning to brown and release liquid, about 5 minutes. Add the shallots and continue cooking until mushroom mixture dries out, is golden brown and shallots are soft, about 10 minutes. Remove from heat and cool.

Step 3

Preheat to the oven to 400 degrees F. Line a baking sheet with parchment paper or a silicone baking mat.

Step 4

On a work surface, roll 1 sheet of puff pastry to a 10 by 14-inch rectangle. Put teaspoon-sized mounds of mushroom mixture on the pastry, evenly spacing them in 4 rows of Top the mushroom mound with a piece of beef, seared side up. With a sharp knife, cut the pastry into even squares around the meat and mushrooms. Working 1 at a time, pull 2 opposite sides of pastry up over each beef piece, then fold the ends over the top to make a packet. Invert and arrange the packets seam side down on the baking sheet and press them lightly to seal the pastry. Repeat with the remaining beef, mushrooms, and pastry.

Step 5

Bake the Wellingtons until golden brown, 20 to 25 minutes. Remove from the oven to a serving platter and let cool at least 10 minutes before serving.