Preheat your oven to 400°F (200°C) and prepare a baking sheet lined with parchment paper.

In a nonstick skillet, melt 4 tablespoons of butter over medium heat. Add the sliced leeks and sauté until softened and translucent (about 8–10 minutes).

Add cremini and shiitake mushrooms, seasoning with salt and pepper. Cook until the mushrooms release their juices (about 12–15 minutes). Let cool slightly.

Combine the mushroom and leek mixture with minced garlic, grated Parmesan cheese, and chopped parsley in a mixing bowl. Stir gently until blended.

Layer 4 sheets of phyllo pastry on a surface, brushing each with melted butter. Place ¼ of the filling along one edge, roll tightly, and seal with beaten egg.

Brush the tops of assembled strudels with melted butter and place on the prepared baking sheet.

Bake for about 15–20 minutes, watching for a golden-brown color.

Let cool for 15 minutes, slice into pieces, and serve warm.