Asparagus and Celery Soup

It is difficult to offer an exciting photo for this soup as it is just....green.  I could have tarted it up with swirls of cream and artfully arranged celery leaves of asparagus spears, but why bother?  It is just simply a delicious, asparagusy and celeryish plate of goodness (are those real words?).  It is made in the way of most blended veg soups: sweat chopped asparagus (about 400g of English green asparagus), a head of celery, chopped (include the leaves if you have any) and a large onion or leek, chopped, in a tablespoon of olive oil and 25g of butter.  Keep the heat moderate and the lid on, so they gently soften and give off their own juices, and we don't want them to brown.  This soup needs a bit of starch to thicken it, so I stir in a dessertspoon of flour, then just cover with boiling water and some marigold stock powder to taste.  Grind in some black pepper, cover and simmer for about 20 minutes.  Blend with a stick blender or processor, return to the pan, and thin down, if liked, with creamy milk or some creme fraiche.  Lovely with crusty bread and cheese for lunch or supper and enough for four.