Heat the oil in a large saucepan, add the onion and fry for 5 minutes until just beginning to soften. Stir in the garlic, fennel seeds and spices and cook for 1 minute to release their flavour.
Add the root vegetables, stock, salt and pepper and bring to the boil. Cover and simmer for 45 minutes, stirring occasionally until the vegetables are very tender. Allow to cool slightly, then purée in batches in a blender or food processor until smooth.
Pour the purée back into the saucepan and stir in the milk. Reheat then taste and adjust the seasoning if needed. Ladle into bowls and serve with croûtons (see