Fry onion in butter over medium heat. Add flour and lightly brown, then add milk and stock and let simmer for ten minutes. Add garlic and simmer for an additional five minutes. (At this point you could puree with an immersion blender or strain for a smoother soup.).
Before serving the soup, add the sour cream and stir it altogether. Toast the croutons or baguette toasts and serve them together with the soup.