|2||pounds fresh spinach, stemmed and well rinsed|
|Butter (for the dish)|
|4||tablespoons ( 1/2 stick) butter|
|1/2||onion, finely chopped|
|Salt and pepper, to taste|
|1/4||cup grated Parmesan|
1. In a large flameproof casserole bring the water to a boil. Add the spinach, turn down the heat and cook, turning the leaves with tongs, for 4 minutes or until the greens wilt. Use tongs to transfer the spinach to a colander. Rinse with cold water. When cool, squeeze the leaves with your hands to remove excess moisture.
2. Turn on the broiler. Butter a 10-inch baking dish.
3. Wipe out the casserole. Add the butter. When it melts, cook the onion, salt, and pepper over medium heat, stirring often, for 5 minutes. Sprinkle with flour and cook, stirring, for 2 minutes.
4. Add the milk 1/2 cup at a time, stirring constantly, until the mixture thickens and all the milk is added. Simmer, stirring, for 2 minutes. Stir the spinach into the sauce. Cook for 2 minutes to reheat the spinach. Taste for seasoning and add more salt and pepper, if you like.
5. Transfer the mixture to the baking dish. Sprinkle with Parmesan. Broil 6 inches from the element for 5 minutes, watching the dish carefully, or until the cheese browns.