Cook spinach according to package directions, with onion slices. Drain mixture. Place mixture and 2 cups chicken broth in container of an electric blender; process until smooth.
Melt butter in a large Dutch oven over low heat; add flour, salt, and pepper, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add remaining chicken broth; bring to a boil, stirring constantly. Stir in spinach mixture; simmer, uncovered, over low heat 10 minutes. Add half-and-half and crab meat, stirring well. Heat thoroughly. (Do not boil.) Stir in nutmeg. Ladle into soup bowls.