heat the oil in a large pan and fry the celery, onion and carrots for 10 mins until softened. Add the potatoes, tomatoes, bouillon, tomato purée and garlic, then stir well. Cover with a lid and cook for 35 mins.
Remove from the heat and blitz until smooth with a hand blender. Spoon half the soup into two bowls, drizzle in the sour cream and scatter with parsley.