Prepare the vegetables and cut them up small. Melt the butter in a large saucepan. Add the vegetables and sugar and cook them gently for 5–6 minutes. Stir in the stock and add the bouquet garni. Bring up to the boil, simmer for 2 minutes and then skim. Cover the pan with a lid and simmer gently for about 40–50 minutes or until the vegetables are quite tender.
Remove the bouquet garni and pass the vegetables and liquid through a soup mill or purée in an electric blender. Return the soup to the saucepan. Measure the milk into a basin and sift the flour over the top. Whisk well to mix thoroughly and then add the blend to the soup. Stir until boiling, then cook gently for 2–3 minutes. Check the seasoning with salt and pepper.
Scatter a little chopped parsley over the top and serve with diced toasted or fried bread.