Spinach Gratin

Spinach is probably my #1 favorite veggie. In my baby book, my mom wrote that I told her spinach was my favorite food when I was only 18 months old. I love spinach quiche, spinach dip, creamed spinach, spinach salad, spinach on my Eggs Benedict, you name it...and this dish definitely does not fail to deliver. Even though it's considered a side dish, I could easily eat it as my main dish. It seems like 3 bags of spinach is a lot, but on the dinner table, my husband and I and our 4 kids will eat all of it, fighting for seconds.

4 tablespoons organic butter
2 organic yellow onions, chopped
1/4 cup organic flour
3 cups organic whole milk
1/4 teaspoon freshly grated nutmeg
pinch of cayenne, to taste
1  1/2 cup grated Gruyere cheese
3 bags of organic, chopped frozen spinach - thawed
1 teaspoon sea salt
Fresh ground black pepper, to taste

Panko Topping
2 tablespoons organic butter
1 1/2 cups panko breadcrumbs
Sea salt, to taste

Preheat oven to 375 degrees. Squeeze the liquid from the thawed spinach, handful by handful. Set aside. Heat milk to simmer in saucepan. 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. Stir in the cheese and season with salt and pepper. Taste and adjust seasoning. Add the spinach and mix in well. Pour into a 9X11 glass or ceramic baking dish. Prepare Panko Topping. Melt butter in medium skillet. Add panko, sprinkle with sea salt and stir until lightly toasted, just a minute or two. Gently smooth the toasted panko on top of the gratin. Bake for 30-35 minutes until bubbling.