Adapted from Gourmet.
3 pounds fresh spinach, stems discarded, or 3 (10-ounce) packages frozen leaf spinach, thawed
5 tablespoons unsalted butter, divided, plus additional for greasing pan
3/4 cup finely chopped onion (1 small)
3 garlic cloves, minced
1 1/2 teaspoons salt
3/4 teaspoon black pepper
Rounded 1/4 teaspoon freshly grated nutmeg
1 cup heavy cream
4 pounds butternut squash (2 large), peeled, quartered, and seeded
1/4 cup finely grated Parmigiano-Reggiano
If using fresh spinach, bring 1 inch water to a boil in a 6- to 8-quart pot over high heat. Add spinach, a few handfuls at a time, and cook, turning with tongs, until wilted, 3 to 5 minutes.
Drain and rinse under cold water. Thoroughly squeeze the cooked or thawed spinach in handfuls to remove excess moisture, then coarsely chop and transfer to a bowl.
Melt 3 tablespoons butter in an 8-inch heavy skillet over moderately low heat. Cook onion and garlic, stirring, until softened, 3 to 5 minutes. Add onion mixture to spinach along with salt, pepper, nutmeg and cream, and stir to combine.
Put oven rack in upper third of oven and preheat oven to 400 degrees. Butter a 3-quart shallow baking dish (13 by 9 inches; not glass). Cut squash pieces lengthwise into 1/8 -inch-thick slices with knife or mandoline slicer.
Layer squash and spinach mixture in baking dish, using about one-fifth of squash and one-fourth of spinach for each layer, beginning and ending with squash.
Sprinkle top layer of squash evenly with cheese and dot with remaining 2 tablespoons butter, then cover directly with a sheet of parchment or wax paper.
Bake until squash is tender and filling is bubbling, 25 to 30 minutes. Remove paper and bake gratin until browned in spots, 10 to 15 minutes, or broil 3 inches from heat, 2 to 3 minutes.
Per serving: 271 calories; 20 g fat (12 g saturated fat; 66 percent calories from fat); 20 g carbohydrates; 64 mg cholesterol; 703 mg sodium; 8 g protein; 17 g fiber.