To make the sauce, melt butter in a saucepan and whisk in the flour. Cook, stirring, for 2 to 3 minutes. Gradually whisk in reserved cooking liquid and enough turkey stock to equal 3 cups, and the cream. Bring to a boil, reduce heat, and continue to simmer for 3 minutes. Season well with salt, pepper and nutmeg. Pour over the turkey and the vegetables.