Spinach Gratin

Ingredients

• Cooking spray or butter, for coating the baking dish
• 3 pounds or 5 (10-ounce) packages frozen chopped spinach (see Recipe Notes if using fresh spinach)
• 6 ounces Gruyère cheese
• 1/2 medium yellow onion
• 5 tablespoons unsalted butter
• 1 1/4 teaspoons kosher salt, divided, plus more as needed
• 1/4 cup all-purpose flour
• 3 cups whole or 2% milk
• 1/4 teaspoon freshly grated nutmeg
• 1/3 cup panko breadcrumbs
• 2 tablespoons grated Parmesan cheese

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Recipe Notes 

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