Saute onion, celery and carrot in oil until soft, about 10 minutes.
Stir in corn, thyme, water and a little salt; simmer 20 minutes.
Coarsely puree mixture; return to pot.
Stir in salt, white pepper and cayenne to taste.
Remove from heat; stir in lemon juice.
Dice half of the avocados and stir into soup. Slice remaining avocado.
Divide soup among warmed soup bowls. Garnish with avocado slices and carrot threads.