Cook the spinach up to 1 day ahead; make the white sauce up to 2 days ahead. To reheat, combine the spinach and white sauce in a saucepan and cook over low heat until they are thoroughly incorporated, then simmer them together as directed below.
2 pounds spinach, washed and stemmed
1 1/2 tablespoons butter
1 1/2 tablespoons flour
1 1/2 cups whole milk
teaspoon freshly grated nutmeg
Salt and pepper, to taste
1/4 cup heavy cream
Pack the spinach into a large saucepan with just the water that clings to it from rinsing. Cover the pan with the lid and set the pan over medium heat. Cook the spinach for 3 minutes or until it wilts completely and is tender.
Drain the spinach into a colander and then rinse with cold water. When the spinach is cool enough to handle, squeeze as much water out of it as possible. You can put the spinach in a food mill for a couple of turns, squeeze it in a clean tea towel (which stains the towel but works well), or press it as hard as you can between two matching plates.
Chop the spinach and set it aside.
In a saucepan, melt the butter over medium heat and whisk in the flour. Cook, stirring constantly, for 2 minutes. Remove the pan from the heat. Gradually whisk in the milk until it is all incorporated.
Return the saucepan to medium heat and bring the sauce to a boil, stirring constantly. Use a large metal spoon to stir in the nutmeg, spinach, salt, and pepper. Cook the mixture for 10 minutes, stirring occasionally. Stir in the cream.
Taste for seasoning and add more salt, pepper, or nutmeg if you like. Serve at once. Sheryl Julian and Julie Riven.