First make the dressing: add all the ingredients to a bowl and stir to combine. Taste and adjust. Set aside.
Roast the corn over an open flame. Roast until charred all over, turning as often as needed. Transfer to a chopping board and let cool slightly.
Meanwhile place the watermelon, pineapple, onions, lettuce and coriander leaves in a large bowl.
Lay the corn down horizontally on your chopping board. Hold the corn firmly with one hand and run a knife along the length of it with your dominant hand. Turn as you go and cut as close to the cob as possible to release the kernels.
Add the kernels to the salad. Gently toss everything together with your fingers making sure not to squish the watermelon. The salad can be prepared and kept refrigerated at this stage until ready to serve.
Dress the salad and sprinkle over the fried onions just before serving. Serve cold or at room temperature.