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1
Make the topping: Combine the crumbs, melted butter and a pinch of salt in a bowl.
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2
Mix in the cheddar and thyme.
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3
Prepare the gratin: Heat the oven to 350.
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4
Rub a shallow 2 quart gratin dish with butter.
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5
Cook the leeks: Melt 2 tablespoons butter in a small saucepan over medium heat.
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6
Add leeks and a big pinch of salt.
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7
Cook, stirring frequently, until well softened and lightly browned, 10-15 minutes.
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8
Add 2/3 cup cider and simmer 2 minutes to reduce slightly.
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9
Add 1/2 cup cream, the chopped thyme, a pinch of salt, a few grinds of pepper, and stir well.
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10
Set aside.
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11
Cook the apples: In a large non-stick skillet, melt the remaining 2 tablespoons butter over medium heat.
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12
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.
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13
Add the remaining 3 tablespoons cream and 3 tablespoons cider.
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14
Stir and let the liquids reduce slightly for a few seconds; remove from heat.
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15
Assemble and bake the gratin: Peel the squash neck, cut into quarters lengthwise, and cut them across into thin slices.
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16
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 teaspoons salt.
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17
Using a rubber spatula, mix gently but thoroughly.
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18
Scrape the mixture into the prepared gratin dish.
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19
Cover with the breadcrumb topping.
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20
Bake until the crust is deep golden brown and the squash and potato are tender when pierced with a fork, about 1 hour.
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21
Let rest for 15-20 minutes before serving.
