Thinly slice the onions and saute in the butter with a pinch of salt until soft and golden brown, about 30-40 minutes. Add the garlic and cook for a minute or so then add the flour and stir to coat the onions. Add the chicken broth, thyme, basil and sherry and cook for another 20 minutes or so to let the flavors marry and the soup to thicken up a bit. Add the cream and salt and pepper to taste.Serve with chopped fresh chives and garlic croutons.