Heat oil in a large pot over medium heat. Add onion, carrot and celery; cook, stirring, until soft, about 5 minutes. Add cauliflower, sweet potato, thyme, bay leaf, salt and pepper. Cook, stirring occasionally, for 3 minutes. Sprinkle flour over the vegetables and cook, stirring, for 1 minute. Add broth and milk and bring to a simmer. Adjust heat to maintain a simmer, cover and cook until the vegetables are soft, about 10 minutes.