Cook spinach in 1 inch of boiling salted water in a large pot, covered, stirring once or twice, until wilted, 1 to 2 minutes. Drain in a colander and rinse under cold running water until cool. Squeeze small handfuls of spinach to remove as much moisture as possible, then coarsely chop.
Cook onion in butter in a small heavy saucepan over moderately low heat, stirring, until softened, about 4 minutes. Whisk in flour and cook, stirring, 2 minutes. Add warm milk and simmer, whisking, 3 to 4 minutes. Whisk in Parmesan, salt, nutmeg, and pepper to taste. Add spinach and cook, stirring, until heated through.
Recipe courtesy of Gourmet Magazine