This is a wonderful dish incorporating a number of winter/fall type vegetables. It takes some time to prepare but the results make it worthwhile.
For the topping:
1-1/2 cups coarse fresh breadcrumbs
2 Tbs. unsalted butter, melted
3 oz. (3/4 cup) grated sharp Cheddar
1 teaspoon fresh thyme leaves
For the gratin:
1/4 cup unsalted butter, plus 1/2 tsp. for the dish
2 cups sliced leeks (white and light green parts of 3 medium leeks), thoroughly washed Kosher salt
2/3 cup plus 3 Tbs. apple cider
1/2 cup plus 3 Tbs. heavy cream
2 tsp. chopped fresh thyme leaves
Freshly ground black pepper
2 crisp, firm apples, (about 14 oz. total), peeled, cored, and thinly sliced
12 oz. butternut squash or other winter squash
2 medium Yukon Gold potatoes (12 oz. total) or whatever kind of potatoes you have
Make the topping:
Combine the crumbs, melted butter, and a pinch of salt in a bowl. Mix in the Cheddar and thyme.
Prepare the gratin:
Heat the oven to 350°F. Rub a shallow 2-quart dish with 1/2 tsp. of the butter.
Melt the 2 Tbs. butter in a small saucepan over medium heat. Add the leeks and a big pinch of salt. Cook, stirring frequently, until well softened and lightly browned (the pan will be dry), 10 to 15 minutes. Add 2/3 cup of the cider and simmer for 2 minutes to reduce it slightly. Add 1/2 cup of the cream, the chopped thyme, a pinch of salt, a few grinds of pepper, and stir well; set aside.
Cook the apples: In a large nonstick skillet, melt the remaining 2 Tbs. butter over medium heat. Turn the heat to medium high, add the apple slices, and cook, gently flipping and stirring, until most of the slices are browned and limp but not falling apart, about 10 minutes. Add the remaining 3 Tbs. cream and 3 Tbs. cider. Stir and let the liquids reduce slightly for a few seconds; remove from the heat.
Assemble and bake the gratin:
Peel the squash, cut it into quarters lengthwise, and cut them across into thin slices. Peel the potatoes, cut them in half, and cut them across into thin slices. In a large bowl, combine the squash slices, the potato slices, the leek mixture, the apple mixture (scrape the pans well), and a scant 2 tsp. salt. Using a rubber spatula, mix gently but thoroughly. Scrape the mixture into the prepared gratin dish, smoothing and pressing until evenly distributed. Cover with the breadcrumb topping.
Bake until the crust is deep golden brown, and the crust is dark brown around the edges, about 1 hour. Let rest for 15 to 20 minutes before serving.
Adapted from Fine Cooking.com