Vegetarian Pastitsio with Lentils and Mushrooms

Prepare the Lentil and Mushroom Filling

1. Heat olive oil in a large pot or Dutch oven over medium heat. Add chopped onion and cook until softened, about 5-7 minutes.
2. Add minced garlic and chopped mushrooms. Cook until mushrooms release their liquid and it evaporates, about 8-10 minutes.
3. Stir in the rinsed lentils, crushed tomatoes, vegetable broth, cinnamon, and allspice. Season with salt and pepper to taste.
4. Bring the mixture to a boil, then reduce heat and simmer, covered, for 30-40 minutes, or until lentils are tender and the sauce has thickened. Stir occasionally.

Prepare the Béchamel Sauce

1. In a saucepan, melt butter over medium heat. Whisk in flour and cook for 1-2 minutes to form a roux.
2. Gradually whisk in the warm milk until smooth. Bring to a gentle simmer, whisking constantly, until the sauce thickens.
3. Remove from heat. Stir in nutmeg and grated parmesan cheese (if using). Let cool slightly, then whisk in the egg yolk.

Assemble and Bake

1. Preheat oven to 180°C (350°F).
2. In a large bowl, combine the cooked pastitsio pasta with about half of the béchamel sauce. Mix well.
3. Spread the pasta mixture evenly in the bottom of a large baking dish (approximately 9x13 inches).
4. Spoon the lentil and mushroom filling evenly over the pasta layer.
5. Pour the remaining béchamel sauce over the filling, spreading it to cover completely. Sprinkle breadcrumbs evenly over the top.
6. Bake for 30-35 minutes, or until the top is golden brown and bubbling. Let stand for 10-15 minutes before serving.