Preparation

If using fresh spinach, bring 1 inch water to a boil in a 6-8 quart pot over high heat.Cook spinach 3-5 minutes. Drain well in a colander. Thoroughly squeeze spinach in small handfuls to to get it absolutely dry, then coarsely chop and transfer to a bowl. Melt 3 tablespoons butter in an 8-inch heavy skillet over medium low heat, then 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°F Butter a 3-quart shallow baking dish (13 by 9 inches; Cut squash to separate bulb section from solid neck section, then cut pieces lengthwise into 1/8-inch-thick slices with slicer or food processor. To make the squash easier to cut, microwave for 3-5 minutes. 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 each layer of squash evenly with cheeses and dot with remaining 2 tablespoons butter, then cover directly with a sheet of parchment or wax paper(or use glass lid). Bake until squash is tender and filling is bubbling, 35 to 45 minutes. eRemove paper and bake gratin until browned in spots, 10 to 15 minutes, or broil 3 inches from heat, 2 to 3 minutes.