1.Remove the husks and silk from the corn cobs, then cook the corn in a saucepan of boiling water for 10-12 minutes. Lift out corn and cool; reserve the cooking water.
2.As soon as the corn is cool enough to touch, slice off the kernels with a sharp knife (reserve the kernels). Return the cobs to the pan of cooking water and bring back to the boil.
3.Boil gently for 30 minutes, then discard the cobs. This is the stock.
4.Meanwhile, put the onion, garlic, ginger and oil in a saucepan. Cook gently until the onion is tender and lightly golden, stirring occasionally; don’t let the onion burn.
5.Stir in the allspice and cumin, then pour in 1 litre of corn stock and add 1 teaspoon salt. Bring to a gentle boil, then cook for 10 minutes.
6.Halve and pit the avocados, remove the skin and chop the flesh. Squeeze over the juice of 1 lime and sprinkle with sea salt. Cut the other lime into wedges.
7.Dish up the soup and add a tumble of corn and avocado to each bowl. Sprinkle with coriander and sliced red chilli, and serve with lime wedges.