Heat a large pot with the oil. Add the onion, celery and cook about 4 min., stirring occasionally. Add the garlic and stir for 30 s.
Add the purple potatoes, cauliflower, broth. Cover the pot and bring to the boil. Reduce the heat to low and simmer the soup until the veggies are fork tender.
Sprinkle with salt, pepper, lemon juice. Blend the soup until puree. Add the cream, stir to combine. Let the soup sit for about 5 minutes.