Spinach Gratin | Peachy the Magazine

Ingredients

• 4 tablespoons (½ stick) unsalted butter
• 4 cups chopped yellow onions (2 large)
• ¼ cup flour
• ¼ teaspoon grated nutmeg
• 1 cup heavy cream
• 2 cups milk
• 3 pounds frozen chopped spinach, defrosted (5 10-ounce packages)
• 1 cup freshly grated Parmesan cheese
• 1 tablespoon kosher salt
• ½ teaspoon freshly ground black pepper
• ½ cup grated Gruyère cheese