Several years ago, I found a recipe by Mark Bittman where he roasted thinly sliced whole lemons along with leeks and chicken thighs. I was skeptical, but I tried it. The lemon peels caramelize as they roast, while also slow-releasing their juices to be absorbed slowly by the vegetables. The result is amazing. Just remember to remove all the seeds from the lemons, as those will retain their intense, unpleasant bitterness.
This recipe substitutes Sweet Potatoes for chicken, and the lemons add a wonderful contrast to their sweetness. You don’t need to put any cheese on this, but it does help hold the casserole together better.
Cut 2-3 unpeeled Lemons in quarters and carefully remove all the seeds with a sharp knife. Try to avoid losing too much juice. Place each quarter cut-side down and then very thinly slice them along the other cut side.
Trim and then clean 2 Leeks. Cut in half lengthwise, then cut into 2″ pieces. Thinly slice each piece into julienne strips.
Cover the bottom of a large cast-iron skillet with a generous amount of olive oil. Arrange the leeks in a thin layer on the bottom, then make a single layer of lemon slices. Sprinkle with salt and pepper, plus a few fresh or dried thyme leaves.
Wash 2 lbs. sweet potatoes, then thinly slice into rounds and some half rounds. Layer the sweet potatoes over the leeks so that they are overlapping and completely covering them. Make another layer if you have some left. Sprinkle with more salt and pepper and drizzle with 1 t. olive oil.
Top with grated sharp cheese of your choice. Cover the pan with foil and baking for 30 minutes at 400, then remove the foil and continue baking until the top is nicely browned.