1 cup (250 millilitres) finely chopped leeks, white part only
2 teaspoons (10 millilitres) minced garlic
4 cups (1 litre) chopped thyme leaves
2 tablespoon (5 millilitres) chopped thyme leaves
2 tablespoons (30 mL) chopped flat-leaf parsley
½ cup (125 millilitres) crumbled feta cheese
Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius) and line a baking sheet with parchment paper.
Toss potatoes with oil and thyme leaves, and season well with salt and pepper. Place in a single layer on baking sheet. Bake until fork-tender, about 15 minutes, turning potatoes over halfway through baking time. Set aside.
In a large saucepan over medium heat, melt butter. Add flour and cook until blended, stirring constantly. Whisk in milk, salt and pepper and cook until sauce begins to thicken, then whisk in pumpkin and nutmeg until blended. Remove from heat.
Transfer ½ cup (125 millilitres) béchamel to a small bowl and whisk in egg yolks one at a time. Whisk yolk mixture back into white sauce until blended. Stir in Parmesan. Keep warm.
In a large skillet over medium heat, add one tablespoon (15 millilitres) oil, lamb, cinnamon and nutmeg. Cook lamb until browned, about 15 minutes, breaking meat up with the back of a spoon if necessary.
With slotted spoon, transfer lamb mixture to a bowl. Discard any fat from skillet.
Return skillet to medium heat and add remaining one tablespoon (15 millilitres) oil and leeks. Sauté until leeks are translucent, about five minutes, stirring frequently. Add garlic, chard and thyme and cook until chard has wilted, about one minutes, stirring frequently. Stir in parsley and reserved lamb mixture until combined. Set aside to cool slightly.
Assembly and baking
Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius) and butter a two-quart (two litre) casserole dish.
Arrange Sweet Potato Planks over bottom of casserole, top with lamb mixture and sprinkle with feta. Spoon warm Pumpkin Béchamel over top and smooth with the back of a spoon.
Bake until thoroughly heated and golden brown, about 25 minutes. Let moussaka cool for 10 minute before serving.