Butternut Squash and Creamed-Spinach Gratin

INGREDIENTS

• 5 tablespoons unsalted butter plus additional for greasing pan
• 3 garlic cloves, minced
• 4 pounds butternut squash (2 large), peeled, quartered, and seeded
• 3 pounds fresh spinach, stems discarded, or 3 (10-ounce) packages frozen leaf spinach, thawed
• 3/4 teaspoon black pepper
• 3/4 cup finely chopped onion (1 small)
• 1 1/2 teaspoons salt
• Rounded 1/4 teaspoon freshly grated nutmeg
• 1/2 ounce finely grated Parmigiano-Reggiano (1/4 cup)
• 1 cup heavy cream