Putting sauteed leeks and fennel on the bottom of a baked gratin renders them down into a fragrantly savory caramel. The moisture in the fennel helps the leeks cook without burning. The herbs in this dish are critical to balance the overall sweetness.
Cut the tops off 2 leeks and then slice the shanks in half lengthwise. Rinse them carefully to clean, then cut in 2″ pieces and finally slice them in thin strips.
Cut the tops off 1 fennel bulb and cut the bulb in half. Remove the tough core at the bottom, then separate the sheaths of the bulb and rinse. Thinly slice the sheaths, either crosswise or lengthwise. If you want to enhance the fennel flavor of the dish, snip off some of the fronds from the tops and reserve for later in the recipe.
Heat 2 T. olive oil in a large cast iron skillet and add the leeks and fennel plus 1 T. fresh rosemary leaves, chopped roughly. Saute over medium heat for 10 minutes, until they are completely soft. Add 1 minced clove of garlic and cook for another minute.
Preheat the oven to 375.
Cut 2-3 Delicata Squash in half lengthwise and use a spoon to remove the seeds. Slice the delicata crosswise in quarter-inch thick slices. Toss the squash in 1 T. olive oil and 1 t. salt.
Layer the squash slices over the sauteed vegetables, fitting them together as best as possible. Sprinkle the first layer with salt, pepper and 1/2 t. fresh thyme leaves. Make a second layer.
Grate Gruyere or other sharp cheese to make 1 C. and sprinkle it over the squash.
Bake until the top of the gratin is lightly browned and bubbly, about 30 minutes. If you wish, sprinkle the reserved fennel fronds over the top.
You can make this gratin without the cheese, but you will have to cover it with foil for the first 20 minutes, then uncover it to brown the squash.