To make the soup, in a medium sauce pan, simmer a bone-in chicken breast in broth and water for 1-2 hours with lemongrass and bay leaf. Remove chicken from pot, pull meat parts away from bones and set aside. Discard the bones. Add chopped roasted celeriac, carrot, a whole lemon cut in half, green onions, and the leek. Simmer until vegetables are fully cooked, and then remove the whole lemon pieces. Using an immersion blender, blend the cooked vegetables, broth, fresh cilantro, and a small lemon slice. Sir in the cut chicken and season with salt and pepper to taste.