1. Roast chestnuts in oven at 200℃ for 20 minutes, remove skin and set aside.
2. Discard top, green part of leek, finely shred the lower white part; dice celery; slice shallot.
3. Melt butter in skillet, sauté shallot, leek and celery until fragrant, turn to medium heat, continue sautéing until ingredients soften, pour water in, bring to a boil in high heat, add thyme.
4. Turn to medium heat, add chestnuts and cream, bring to a boil then turn to low heat, cook until chestnuts soften.
5. Turn to medium heat, remove thyme sprigs, take a few chestnuts out mince chop coarsely, set aside. Blitz the rest of the ingredients with a hand blender.
6. Season with salt and pepper. Place chopped chestnut in bowls, pour soup in, drizzle with olive oil, serve.