Heat the oil in a frying pan, add the spring onions and fry for 5 minutes until softened. Cut very thin strips from the remaining spring onions to create curls. Soak in cold water for 10 minutes, then drain.
Scoop out the avocado flesh from the shells with a dessertspoon and add to a blender or food processor with the fried spring onions, the soured cream and about a third of the stock. Blend until a smooth purée, then gradually mix in the remaining stock and lime juice. Season to taste with salt, pepper and a few drops of Tabasco sauce.
Serve the soup immediately while the avocado is still bright green in cups or glass tumblers containing some ice. Scatter the spring onion curls over the soup and serve with breadsticks or grissini.