01 -
Heat the olive oil in a heavy skillet over medium-high heat. Season the beef cubes with salt and pepper, then sear them for 2 to 3 minutes per side until browned on all surfaces. Remove the beef from the skillet and allow to cool.
02 -
In the same skillet, add the chopped mushrooms, shallots, and minced garlic. Sauté for 5 to 7 minutes until the mushrooms are golden brown and liquid has evaporated. Stir in thyme, season with salt and pepper, and let the mixture cool completely.
03 -
Dust a clean work surface with flour. Roll out the puff pastry sheets to a thickness of approximately 1/8 inch. Slice into 3 x 3 inch squares using a sharp knife or pastry cutter.
04 -
Spoon a portion of the cooled mushroom duxelles onto each pastry square. If desired, spread a thin layer of Dijon mustard on top. Place one seared beef cube onto each mound, then fold the pastry over to enclose, pressing and sealing the edges with a fork.
05 -
Preheat oven to 400°F. Arrange assembled bites on a parchment-lined baking sheet, seam side down. Brush the tops lightly with beaten egg and bake for 15 to 20 minutes, or until the pastries are golden brown.