In large pot over medium heat, melt butter; add onion and sauté about 8 minutes or until soft and translucent. Add flour; cook, stirring, 2 minutes. Gradually pour in stock, stirring until smooth. Add spinach and salt; bring to a boil. Reduce heat; cover and simmer 10 minutes, stirring occasionally. Stir in soymilk and nutmeg. Cook over low heat until heated through.