Heat the oil in a large pan and add the onions, potatoes, and leeks. Cook for 3-4 minutes.
Add the vegetable stock, salt, and pepper. Simmer until the vegetables are soft and tender.
Mix in a blender until you have a smooth mixture.
Reheat in a clean pan, stir in the cream or crème fraîche, and heat through.
Serve with some nice crusty bread.