Vegetarian Lentil Moussaka

Wash your hands with soap and water and clean the work surface. Heat the oven to 425°F, with racks in the upper and lower thirds. Rinse and dry the vegetables, scrubbing the potatoes and removing any damaged areas. Trim the eggplant and cut it into 1/2-inch-thick rounds. Slice the unpeeled potatoes into 1/4-inch-thick rounds. Peel and finely dice the onion, then peel and mince the garlic.

Pick over the lentils, rinse them in the strainer, and place them in one saucepan with the 4 cups water. Bring to a boil, then reduce the heat and simmer gently, partially covered, for 25–35 minutes, until tender but still holding their shape. Drain thoroughly. Prepare the vegetables and tomato filling while the lentils cook.

Place the eggplant on one baking sheet and the potatoes on the other. Coat the eggplant evenly with 2 tablespoons olive oil and 1/4 teaspoon salt. Toss the potatoes with 1 tablespoon olive oil and 1/4 teaspoon salt. Arrange both vegetables in single layers. Roast for 25–35 minutes, turning the slices and switching the sheets between racks halfway through. Remove each sheet when its vegetables are lightly browned and completely tender. Reduce the oven temperature to 400°F.

Meanwhile, heat 1 tablespoon olive oil in the skillet over medium heat. Add the onion and cook for 6–8 minutes, stirring occasionally, until softened and beginning to turn golden. Add the garlic, cinnamon, and oregano and stir for 30 seconds. Add the crushed tomatoes, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Simmer uncovered over medium-low heat for 12–15 minutes, stirring occasionally, until thickened and no watery liquid collects around the edges.

Melt the butter in the second saucepan over medium-low heat. Whisk in the flour and cook, whisking continuously, for 2 minutes without browning it. Gradually whisk in the milk, adding a small amount first and whisking until smooth before adding more. Bring to a gentle simmer over medium heat, whisking frequently. Reduce the heat and cook for 4–6 minutes, until smooth, thick, and spreadable. Stir in the remaining 1/2 teaspoon salt, remaining 1/4 teaspoon black pepper, and nutmeg. Remove from the heat.

Grease the baking dish with the remaining 1 teaspoon olive oil. Arrange the roasted potatoes across the bottom, overlapping them slightly. Spread half the lentil filling evenly over the potatoes. Add the roasted eggplant in an even layer, then spread the remaining lentil filling over it. Spoon the béchamel over the entire dish and gently spread it to the edges.