Heat grill to medium hot. Brush tomatoes and onion with oil and place, with the corn, on the grill.
Grill 10 cm from heat, turning frequently, until lightly browned, 10 to 12 minutes.
When cool enough to handle, finely chop tomatoes and onion. Remove husk from corn and cut kernels from cob.
Combine all the ingredients in a serving bowl. Refrigerate until ready to serve.
Helpful hint
Coriander's distinctive flavour does not please every palate. If it is too intrusive for you, try dill, chervil or Italian flat-leaf parsley in your salsa.