This amazing side is based on an original recipe on epicurious and is a fantastic pair for cold weather comfort food!
3 (10-ounce) packages frozen leaf spinach, thawed
5 tablespoons unsalted butter
3/4 cup chopped onion
3 garlic cloves minced or crushed
1 1/2 teaspoons salt
3/4 teaspoon black pepper
Rounded 1/4 teaspoon freshly nutmeg
1 cup heavy cream
2 large butternut squash – peeled, quartered, and seeded
1/4 cup finely grated Parmigiano-Reggiano
Preheat oven to 400 degrees.
Squeeze thawed frozen spinach in small 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, then cook onion and garlic for 3-5 min stirring periodically. Add onion mixture to spinach along with salt, pepper, nutmeg, and cream and stir to combine.
Cut squash to separate bulb section the straight neck section, then cut pieces in long strips into 1/8-inch-thick slices. Take good care as butternut squash is awkward and easy to cut yourself.
Layer squash and spinach mixture in baking dish 13 x 9, using about 1/5 of squash and 1/4 of spinach for each layer, beginning and ending with squash. Sprinkle top layer of squash evenly with cheese and add remaining 2 tablespoons of butter, then cover directly with wax paper. Bake 25 to 30 minutes until squash is tender and filling is bubbling. Uncover and bake gratin another 10-15 min until browned slightly.