Spinach Gratin

INGREDIENTS

• 4 tablespoons (1/2 stick) unsalted butter
• 1/4 cup flour
• 1/2 teaspoon freshly ground black pepper
• 1/4 teaspoon grated nutmeg
• 3 pounds frozen chopped spinach, defrosted (5 (10-ounce) packages)
• 2 cups milk
• 1/2 cup grated Gruyere cheese
• 1 cup heavy cream
• 1 tablespoon kosher salt
• 1 cup freshly grated Parmesan cheese
• 4 cups chopped yellow onions (2 large)