Heat a large saucepan, add oil then garlic, cook for 30 secs – 1 minute, stirring to prevent garlic burning. Add butter, allow to melt then add onion, celery, leek and thyme; cook until softened, about 5 -10 minutes. Add the cauliflower and potato, gently cook for 5 minutes then add the stock. Bring to the boil then reduce heat and simmer for about 20 - 30 minutes until the cauliflower is soft and the potato almost collapsing. Add cream if using and 2/3rds season with salt and pepper.