4 T. unsalted butter
4 c. chopped yellow onions (2 large onions)
1/4 c. flour
1/4 tsp. grated nutmeg
1 c. heavy cream
2 c. milk
3 lb. frozen, chopped spinach, thawed* (5 10 oz. packages)
1 c. freshly grated Parmesan cheese
1 T. kosher salt
1/2 tsp. black pepper
1/2 c. grated Gruyere cheese*
Preheat oven to 425 degrees. Melt butter over medium heat. Add onions and cook until translucent, about 15 minutes. Add flour and nutmeg. Cook, stirring, for 2 minutes. Add cream and milk. Cook until thickened.
Squeeze as much liquid as possible from spinach*. Add spinach to sauce. Add 1/2 c. Parmesan cheese and mix well. Season with salt and pepper. Pour into a 2 quart baking dish (I used an 8x8, but if you have some kind of round or oval dish near that size, that would be great too). Sprinkle remaining cheese on top.
Bake at 425 degrees for 20 minutes, until hot and bubbly.
*A few notes: Allow plenty of time for your spinach to thaw and then really squeeze all the liquid out it. I set my spinach in a colander over a bowl to thaw on the counter and it took a good 14-16 hours to really thaw. Then I pushed down on it with a spoon to squeeze out the liquid and then, I stuck it all in a dish towel and rang it out. A LOT of liquid comes out of there, which is good.
Also, if you don't want to break the bank for the Gruyere cheese, you can use swiss - it's very similar in taste. My sister says she always just uses more Parmesan, so that's an option too.
Source: My sister Wendy, originally from Barefoot Contessa