Melt the butter in a saucepan and add the onion, sweetcorn, bay leaf and peppercorns. Cook gently for 5 minutes, then stir in the stock. Bring to the boil, cover and simmer for 25 minutes.
Remove the bay leaf and peppercorns and sieve the soup. Return it to the saucepan. Dissolve the cornflour in the milk and add to the pan with salt and pepper to taste. Bring to the boil, stirring and simmer for 8-12 minutes.
* I personally don't discard the sweetcorn. I pulse it in the food processor and add it back to the soup for the final simmer as it helps thicken and adds even more taste.