Cook the sweet potatoes and butternut squash separately in boiling salted water for 15 minutes or until just tender. Drain both well.

Meanwhile, melt half the butter in a large, heavy-based saucepan. Add the onion and cook for 4-5 minutes, until softened but not browned. Add the haddock fillets and water.

Bring to the boil, reduce the heat and simmer for 10 minutes, until the fish is cooked. Use a draining spoon to lift the fish out of the pan and leave to cool. Set the cooking liquid aside.

When cool enough to handle, carefully break the flesh into large flakes, discarding the skin and bones. Set the fish aside.

Press the sweet potatoes through a sieve and beat in the remaining butter with seasoning to taste. Strain the reserved fish cooking liquid and return it to the rinsed-out pan, then whisk in the sweet potato. Stir in the milk and bring to the boil. Simmer for about 2-3 minutes.

Stir in the butternut squash, fish, Thai basil leaves and cream. Season the soup to taste and heat through without boiling. Ladle the soup into six warmed soup bowls and serve immediately.