Melt the butter in a large saucepan, add the onion and celery, saut� until soft. Add most of the milk and all of the stock, simmer for 20 minutes.
Once cooked, liquidise the soup until smooth. Return to the saucepan and bring back to the boil. Blend the cornflour with the remaining milk, stir into the soup, simmer for 5 minutes.
Reduce the heat, stir in the Stilton. Pour into soup bowls and garnish with a swirl of cream.