First, place the mushrooms, shallot, thyme, a pinch of salt and freshly ground black pepper in a food processor, and pulse until finely ground.
Transfer the mushroom mixture to a dry non-stick skillet, and saute until it starts to dry out and darken slightly. We’re cooking the water out of the mushrooms so it doesn’t make the puff pastry soggy.
When the mushroom mixture is done, place it in a small bowl and set aside.
Meanwhile, sprinkle the beef pieces evenly with salt and pepper. Then, coat the beef pieces in flour.
Add the olive oil to a cast iron (or similar) skillet, and heat it over high heat for about 4-5 minutes. Add the beef to the skillet and sear it on one side very quickly; do not worry about cooking the beef all the way through. Move the beef to a plate to cool (yes, we’re only searing it on one side to prevent over-cooking).
Next, get ready to assemble your beef parcels:
Have the mustard ready in a small dish with a pastry brush.
Cut each prosciutto slice into 3 pieces with scissors.
Keep your puff pastry in the fridge until ready to use.
Lay out all 15 slices of prosciutto, and brush them lightly with mustard. Next, sprinkle some of the mushroom mixture on each slice.
Place a piece of beef on each parcel, and lightly pinch it closed into a little pouch. Place each pouch into the mini muffin cups, and refrigerate for an hour.
A few minutes before the one hour timer is up, preheat the oven to 425.
Unroll the puff pastry sheet, and cut it into 15 squares.
Remove the muffin tin from the fridge, and move each prosciutto parcel onto a square of puff pastry. (While the muffin pan is empty, take the time to spray it very well with cooking spray, if it’s not silicone. A silicone pan really is the best pan to use here). Pull the sides of the puff pastry up and around the prosciutto parcel, and then place it back in the muffin cup.
Beat the egg yolk with a splash of water, and brush all over the surface of the pastry.
Bake for 15-18 minutes, until golden brown and bubbling.
While the pastries bake, stir together the Cashel blue cheese, buttermilk and sour cream until smooth.
Let the pastries cool in the pan for about 5 minutes before moving them to a serving tray. Dollop each parcel with some of the Cashel blue cheese sauce, and serve.