In a large jug, make up 1.2 litres vegetable stock using the stock cube.
Peel the butternut squash and sweet potato and cut into chunks. Deseed the peppers and roughly chop. Set aside.
Heat the oil in a large pan and cook the onion and garlic for 10 minutes until soft. Stir in the butternut squash, sweet potato, peppers and sun-dried tomato puree. Cook for 10 minutes, until the veg begins to colour.
Pour in the stock and bring to the boil. Cover with a lid and cook over a medium heat for 30 minutes, until the veg is soft.
Pour the soup into a food processor (in batches) and blend until smooth. Divide between 4 bowls and garnish with a sprinkle of toasted seeds to serve.