Pour the cold water into a small saucepan, sprinkle in the gelatine and leave to soak for 5 minutes, then stir over low heat until the gelatine has dissolved (do not boil). Draw off the heat and add the stock. Set aside to cool.
Halve the avocados and remove the stones, then scoop out the flesh with a spoon. In a mixing bowl, mash the avocado flesh with a fork. Add onion juice (twist an