1½ lbs. russet potatoes, peeled and cut into ¼-inch thick slices
4 Tbsp. olive oil, divided
1½ tsp. kosher salt, divided
1½ lbs. small eggplants, skin on and cut into ¼-inch thick slices
1 medium yellow onion, diced
2 tsp. minced garlic
1¼ lb. whole cremini mushrooms, coarsely chopped
1 cup prepared green or black lentils
¼ cup tomato paste
1 tsp. ground cinnamon
¾ tsp. ground nutmeg, divided
1 Tbsp. sugar
½ tsp. freshly ground black pepper
1 28-oz. can diced tomatoes
4½ cups whole milk
1 fresh bay leaf
8 Tbsp. (1 stick) unsalted butter
1 cup flour
4 eggs, beaten
6 oz. crumbled feta or goat cheese
• Preheat the oven to 350 degrees. In a medium bowl, toss the potatoes with 1 tablespoon olive oil and ¼ teaspoon kosher salt. Arrange the potatoes in a single layer on a rimmed baking sheet. Repeat the process with the eggplants, arranging the oiled slices on a second baking sheet. Roast 15 minutes, switching the pans on the oven racks halfway through. Set aside.
• In a heavy skillet over medium-high heat, warm the remaining 2 tablespoons olive oil. Add the onion and cook until soft, about 5 minutes. Add the garlic and cook an additional 30 seconds. Add the chopped mushrooms and cook until the liquids release and evaporate, about 10 minutes.
• Stir in the lentils, tomato paste, cinnamon, ½ teaspoon nutmeg, sugar, remaining 1 teaspoon salt and pepper until well combined. Add the tomatoes with the liquid and reduce heat to medium. Cook 5 minutes until the flavors are well blended. Taste, adjust the seasonings, remove from heat and set aside.
• In a medium saucepan over medium heat, combine the milk, the remaining ¼ teaspoon nutmeg and the bay leaf. Cook until hot, but not boiling.
• In a large saucepan over medium heat, make a roux by melting the butter. Whisk in the flour and cook 2 minutes. Remove the bay leaf from the milk mixture, then add the milk to the roux 1 ladleful at a time, incorporating each addition before adding the next. The sauce will be very thick. Remove the saucepan from the heat and allow to cool slightly. In a small bowl, combine the eggs and feta, then whisk into the white sauce.
• Spray a 10-by-16-inch baking dish with cooking spray. Spread 1 cup white sauce on the bottom. Add the potatoes in a single layer followed by eggplant in a single layer. Spread the mushroom-lentil sauce over the eggplant, then cover with the remaining white sauce. Bake, uncovered, 1 hour or until golden brown. Let cool 15 minutes before serving.