Dredge loin in flour and shake off any excess. Dredge in eggs and then roll in the dried mushroom dust. In a hot skillet, add canola oil to coat the bottom, and sear each fillet over high heat until seared on all sides. Set aside.
In a pot of boiling, salted water, blanch spinach for about 1 minute, until leaves are tender. Strain and immediately place in a bowl with ice and water to stop the cooking process. Squeeze out any excess water from the spinach with your hands and place in a food processor. Add Parmesan, heavy cream, butter, and salt and pepper, to taste. Process until mixture is smooth and resembles a course paste.
Spread the spinach mixture on 1 (16-inch) sheet of plastic wrap. The mixture should be 1 inch from the edge and about 8 inches long. Place seared loin on the far end of the spinach and use the plastic wrap to surround the rest of the meat with the spinach. Twist the ends to make the roll tight. Place in the refrigerator for 1 hour to set the spinach. Remove puff pastry from the freezer to thaw.
Preheat oven to 375 degrees F.
Roll out puff pastry until 1/4-inch thick. Place as many rolls as you can side by side on the sheet and cut pastry in between them (you will have the long strips from the width of the rolls). Roll each piece of pastry around the roll, leaving the twisted plastic sticking out the side, like candy. Roll until the pastry overlaps and cut off any excess. Brush the end with egg wash and seal tightly. Press the pastry firmly around the roll and squeeze the ends around the plastic. Prick pastry with a fork in several places. Place on a sheet pan or cookie sheet lined with parchment paper and place in the oven for 15 to 20 minutes, until puff pastry is golden brown. To make sure meat is cooked to your liking, cut in half and check the doneness. If the meat is too rare, place the meat back in the oven for a few more minutes. Remove from oven and let rest for 5 minutes.
Cut each steak in half. Hold on to the bundle as you pull the plastic wrap out the side. To do this, grab the end of the plastic that was sticking out (the twisted end) and pull. Some of the spinach mixture will be lost but most will still be inside.
In a spice grinder (coffee grinder) grind mushrooms until finely ground. Do not use your regular coffee grinder. Purchase one especially for grinding spices.;