Soften the aromatics: Heat the olive oil in a large saucepan over medium heat. Add the onion, garlic and thyme and cook gently for about 5 minutes, until the onion is soft and translucent without taking on any colour.
Cook the leeks and celery: Add the sliced leeks and celery and continue to cook gently until softened, about 3–5 minutes.
Add the potatoes and spices: Add the potatoes, curry powder and turmeric and stir well to coat everything in the softened vegetables and spices. Season with a little salt and pepper.
Cook the chicken: Pour in the vegetable stock and add the chicken breasts. Bring to a boil, then reduce to a gentle simmer and cover. Cook for about 10–15 minutes, until the potatoes are completely tender and the chicken is cooked through. Remove the chicken and shred it using two forks.
Blend the soup: Blend the leek and potato mixture with an immersion blender until smooth and creamy. Add a little extra hot stock or water if the soup is thicker than you'd like.
Finish and serve: Return the shredded chicken to the soup and stir through the chopped parsley. Add the cider vinegar or lemon juice, then taste and adjust the salt and pepper. Warm gently before serving.