In a large skillet, melt 4 tablespoons of butter over medium heat. Add the sliced leeks and sauté for 8-10 minutes until they become tender and translucent. Remove the leeks from the skillet and place them in a bowl. In the same pan, toss in the sliced cremini and shiitake mushrooms, cooking for 12-15 minutes until their moisture evaporates. Mix these sautéed mushrooms with the leeks, freshly minced garlic, chopped parsley, and parmesan cheese.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper, brushing it lightly with melted butter. Lay out 4 sheets of phyllo pastry on a clean surface, ensuring you brush each layer generously with melted butter to keep it moist and flaky. Once layered, spoon one-fourth of the mushroom filling onto one end of the pastry. Roll the dough tightly around the filling, tucking in the edges to seal securely.

Once your strudels are rolled, place them seam-side down on the prepared baking sheet. Brush the tops with more melted butter to achieve a golden color while baking. Put them in the preheated oven and bake for about 15 minutes, or until the pastry is golden brown and crispy. Keep an eye on them during baking to prevent over-browning.

Once baked, remove the Mushroom Strudel from the oven and let it cool for 15 minutes. This pause allows the filling to set and makes slicing easier. After cooling, slice the strudel into serving pieces. Enjoy it warm, perhaps alongside a fresh salad or your favorite dipping sauce for an elevated experience!