Cook oil and curry powder in a large saucepan over medium heat, stirring, until fragrant, 1 to 2 minutes. Stir in carrots, celery and onion; toss to coat in oil. Cook, stirring frequently, for 10 minutes. Stir in broth. Bring to a boil. Reduce heat and simmer until the vegetables are very tender, about 10 minutes. Remove from the heat; let stand 10 minutes. Lay a paper towel over the surface of the soup to blot away the oil that has risen to the top. Discard the paper towel.
Working in batches of no more than 2 cups at a time, transfer the soup to a blender and puree (use caution when pureeing hot liquids). Return the pureed soup to the pan, place over medium heat and heat through. Season with lemon juice, salt and pepper.