Start with a 50/50 mix of butter and flour roux browned in an iron skillet roughly 45 minutes on med-high heat. Stir almost constantly. If you start with a stick (4 oz) butter and 1/2 cup flour, this roughly will thicken and flavor three thawed ,microwaved and drained boxes of frozen spinach.Once you have stirred the roux into the drained spinach, thin in with milk or cream ( 1/4- 1/1/2 cup) and add salt,pepper and nutmeg to taste. I usually add 1 tsp of each. Keeps well simmering on low heat.

I make a version that calls for onions, spinach, cream cheese, salt, pepper, and a wee bit of nutmeg.

Lizthechef posted a great recipe here for a spinach gratin - http://www.food52.com/recipes/3597_spinach_gratin