Preheat oven to 400 degrees.
Heat oil in a medium nonstick skillet over medium heat. Add mushrooms and Rosemary and sauté 3 minutes or until browned. Season with pepper and salt. Set aside.
Season the pieces of meat with salt and pepper. Place in a heated cast iron pan. Sear the until nice and golden.
Whisk together egg and 1 tablespoon water in a small bowl to make an egg wash.
Spray cooking spray inside the wells of a 12 count muffin pan. Spray cooking spray on the outside of another 12 count muffin tin.
Unroll the puff pastry sheet and place it on top of the muffin tin that you sprayed the inside of. Place the other muffin tin on top of the pastry and push down to make wells in the dough. Squeeze a dollop of mustard inside the center of each well. Sprinkle with thyme.
Place a piece of prosciutto top of the herbs next.
Arrange pieces of beef next. Top each with a spoonful of the crumbled Boursin cheese and some mushroom slices.
Use a pizza cutter to cut between the wells going across and upwards.
Take the corners of the pastry and bring them together and twist to seal to make little bundles.
Brush the tops of the pastry with egg wash. Sprinkle with a little bit of flaky salt.
Place in the preheated oven and bake for 20 minutes or until pastry is puffed and golden brown. Let stand 5 minutes.
Yum!
