In a large saucepan sweat off the diced onion in the butter. Wash, trim and dice the celery, add to the onion and continue on a low heat. Remove and bruises from the apples and grate them into your pan, discarding the core. Cook for a further 10 minutes before adding the stock, seasoning and sugar ~ beware too much salt if you are using a stock cube. Bring to the boil, then simmer, lid on, for 30 minutes don't over cook or the celery flavour becomes jaded and the colours dull. Allow to cool a little before liquidising thoroughly. Pass through a large mesh sieve to ensure there are no stringy threads of celery remaining. Check seasoning and consistency, adding a little milk if needed. Serve in warmed bowls, sprinkled with parsley and the browned almond flakes.