Heat a griddle pan. Cook the corn on the cobs in the hot pan, brushing with the olive oil and seasoning as they cook. Turn the corns now and then, until cooked with nice charred lines, about 10 mins. Once cooked, transfer to a plate and leave to cool.
Halve the avocados and remove the stones. Peel, then cut into chunky pieces. Toss through the half of the lime juice and zest.
Hold the corn firmly on a chopping board and cut away the kernels. Mix the avocado with the sweetcorn and red onion, coriander leaves, a good pinch of salt and the remaining lime juice and zest. Keep chilled until ready to serve.