Step 1

Position a rack in the middle of the oven and preheat to 350 degrees.

Step 2

Peel the squash, use a sharp knife to cut it in half lengthwise, and scoop out and discard (or save for roasting) the seeds and stringy bits. Cut the flesh into 1/4-inch crescents and half-moons. In a large bowl, toss the squash with the cheese, water, cream, 1/2 teaspoon salt and 1/4 teaspoon pepper. Pour into a 6-cup gratin or baking dish, cover tightly with foil, and bake for about 20 minutes, or until the squash starts to soften. Remove the foil and bake for another 15 to 20 minutes, or until the squash is fork-tender.

Step 3

While the squash is baking, peel and core the apples, and cut them into 1/2-inch wedges.

Step 4

In a large skillet over medium heat, melt 3 tablespoons butter. Add the onion and cook, stirring frequently, until it softens and starts to brown, 6 to 8 minutes. Add the rosemary, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper and cook until fragrant, about 1 minute. Add the apples and cook, stirring occasionally, until they soften and start to caramelize but hold their shape, 5 to 7 minutes. Spread the mixture evenly over the squash.

Step 5

Set the oven to broil, and set a rack in the highest position that will allow the gratin dish to sit at least a few inches from the flame or element.

Step 6

In a food processor, pulse the bread with the remaining 1 tablespoon butter, garlic and nutmeg to create coarse breadcrumbs. Sprinkle over the squash and apples. Broil, uncovered, until the topping is golden brown and crunchy, 2 to 3 minutes. Let rest for at least 15 minutes before serving.