- Slice the carrots then place them with parsnips in a roasting tin and season with salt and pepper.
- Roast in oven at 200°C / 400°F / Gas Mark 6, for an hour or until the vegetables are very soft.
- Meanwhile, 20 minutes before the vegetables have finished roasting, put leeks in a large saucepan with stock and a teaspoon of thyme. Simmer covered, for 20 minutes.
- Transfer the roasted root vegetables to a blender or food processor and blend, adding a little of stock if necessary.
- Transfer the processed mixture to saucepan and adjust seasoning. Add remaining thyme, stir and let simmer for 5 minutes.
- Ladle into warmed bowls and serve garnished with thyme sprigs.