Mushrooms can easily mimic meat and this strudel is a perfect example of how blending mushrooms with meat can create a more delicious, nutritious and sustainable meal!
On a lightly floured surface, fold out the first chilled puff pastry sheet and press or roll away any seams. Fold the sheet in half lengthwise and roll out to 15"x 6" (L x W). Place the pastry on a parchment-lined baking sheet, cover with plastic wrap and chill in the refrigerator while the second sheet is prepped.
Repeat for the second sheet of puff pastry rolling it out to 15 inches x 6 inches.
At this point, generously dust the top of the pastry with flour and gently fold in half AGAIN to 15 inches x 3 inches (do NOT press or roll). Using a sharp knife, cut slits in the pastry at 1/2-inch intervals to within 1 inch of the UNFOLDED edge. Place the folded pastry sheet on a second parchment-lined baking sheet, cover with plastic wrap and refrigerate for at least 30 minutes.
Heat a large sauté pan or skillet over medium-high heat. Add the olive oil, mushrooms and onion to the pan and sauté for 5 minutes. Add the thyme and garlic, and cook for one more minute. Do not allow the garlic to brown or burn. Season with salt and pepper.
Spread the mushrooms on a baking sheet to cool and set aside. While the mushrooms are cooling, use the same skillet to brown the beef until no pink is showing, about 4-5 minutes.
Place the beef on a paper towel lined plate and pat off as much grease as possible. Combine the cooked beef with the mushroom mixture and season with salt and pepper to taste. If not using immediately, refrigerate the mixture in a sealed container for up to 2 days.
When ready to assemble, stir the cheese into the cooled beef mixture.
Remove the UNCUT puff pastry sheet from the refrigerator. Distribute the beef, mushroom and cheese mixture evenly down the center of the dough, leaving 1 inch of space around the perimeter. Lightly brush the exposed pastry edges with some of the beaten egg mixture.
Next, lay the CUT pastry over the top so the FOLDED (un-cut) edge lies down the center of the mushrooms. Gently unfold the pastry and pull the edges of the top sheet to meet the edges of the bottom sheet; the slits should be evenly spaced. (Note: This may take a minute or two so just work slowly so as not to tear the pastry and if you DO get a tear, keep working and repair by hand as needed.) Press the outside borders with the tines of a fork to seal.
Refrigerate for 20 minutes, or cover tightly with plastic wrap and refrigerate overnight.
Preheat oven to 425°F. Lightly brush the top of the strudel with the egg mixture.
Bake for 15 minutes, then reduce the oven temperature to 375°F. Continue baking until the strudel is puffed and golden, 15 to 20 minutes more. Serve warm.