Season the beef fillet with salt, pepper, and oil. Sear it on all sides in a hot skillet, then set it aside to cool.

In a saucepan, melt butter and add mushrooms and onions. Cook on high heat until all the liquid evaporates, then add herbs, garlic, and base.

Deglaze with white wine and reduce the liquid. Add heavy cream and continue reducing until dry. Transfer to a food processor and purée until coarsely chopped.

Cut the puff pastry into 6 squares and roll it slightly on a floured surface. Place cheese, duxelles, and meat in the center of the pastry. Fold the corners into the center to cover the meat. Make a hole in the center after turning it over to allow steam to escape during cooking.

Brush the outside with egg wash. Preheat the oven to 425°F and bake for 20-25 minutes or until the pastry is golden brown. Serve with your favourite brown sauce.