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

Directions

1. Preheat oven to 400 degrees.
2. Squeeze the liquid from the thawed spinach, handful by handful. Set aside.
3. Heat milk to simmer in saucepan.
4. Melt the butter in a heavy-bottomed pot. Add onions and cook until wilted but not browned, about 10 minutes. Add the flour and cook for 2 minutes. Stir in the hot milk and whisk the sauce until it thickens. Add the nutmeg and cayenne. Turn off the heat.
5. Stir in the cheese and season with salt and pepper. Taste and correct seasoning. Add the spinach and mix in well.
6. Pour into a 9X11 glass or ceramic baking dish. Prepare crumb topping.
7. Melt butter in medium skillet. Add panko, sprinkle with sea salt and stir until lightly toasted. This occurs quite suddenly.
8. Gently smooth the toasted panko on top of the gratin. Cook for 30-35 minutes until bubbling.

https://food52.com/recipes/3597-spinach-gratin