Spinach Gratin

Ingredients

• Gratin
• 4 tablespoons unsalted butter
• 2 medium-sized yellow onions, chopped
• 1/4 cup unbleached flour
• 3 cups organic whole milk
• 1/4 teaspoon freshly grated nutmeg
• 1 pinch of cayenne
• 1 1/2 cups grated Gruyere cheese
• 3 16 oz. bags of organic, chopped spinach - thawed
• 1/2 teaspoon kosher salt (up to 1 teaspoon)
• 1/4 teaspoon ground white pepper (up to 1/2 teaspoon)
• Panko topping
• 2 tablespoons unsalted butter
• 1 1/2 cups panko breadcrumbs
• 1 pinch sea salt, to taste