Put the onion, leek, carrot, swede or turnip, potato and stock into a large saucepan. Bring to the boil, then reduce the heat and simmer, partially covered, for 25 minutes, until the vegetables are tender.
You could use butternut squash instead of swede or turnip, if you prefer.
Blend the cornflour to a smooth paste with 2-3 tablespoons of the milk. Add the remaining milk to the saucepan with the sweetcorn, then stir in the blended cornflour. Heat gently, stirring constantly, until the soup thickens and just begins to boil.
Season with black pepper and ladle into bowls. Serve each portion with a wholemeal roll.
Cool the soup and store in a covered container for up to 3 days, or freeze for up to 3 months.