Defrost spinach and squeeze out as much water as you can. What I do is put the spinach in a clean dishtowel and wring it out. Set aside.
Heat milk and cream in a saucepan until it's very hot.
Melt the butter in another large saucepan. Add the flour, salt, pepper and nutmeg and whisk until there are no lumps. Cook for 5 minutes to cook the flour.
Slowly whisk in the hot milk/cream. Bring to a simmer and cook for 5 minutes.
Add the drained spinach, stirring to mix well. Simmer for 2 minutes to heat the spinach.
This makes a very thick sauce where a spoon will stand up in it. If you prefer a thinner sauce, use less flour--3 tbs flour will yield a thin sauce that will make the spinach soupier in consistency. Using 4 tbs of flour will make a medium sauce.