First, preheat oven at 400 degrees.
Next, drizzle oil, salt and pepper all over the mushrooms (set aside to marinade).
Now, mix all the ingredients of the stuffing in a a bowl and set aside.
Next, roll out the puff pastry an inch more and divide in to 4 equal squares. Brush Dijon mustard on puff pastry.
Also, divide stuffing evenly on the portobello mushrooms.
Now, add a stuffed mushroom to center of each puff pastry. Place it stuffing side down (see video above).
Next, wrap all the sides of the puff pastry around the mushroom. Make sure to seal in edges buy pinching.
Now, place seam side down on parchment paper line sheet pan. Brush with egg and sprinkle with sesame seeds.
Next, place in fridge overnight or 20 minutes if making tonight (the cold butter steams and puffs the pastry). Then in the oven on middle rack for 20 minutes and enjoy.