First, make the dressing. Put 4 tablespoons of oil into a small bowl, add the salt, cumin, coriander, chilli powder and lemon juice, and set aside.
Cut the courgettes lengthways into 0.5 cm-thick slices. Trim the broccoli, and break bigger branches into individual stems. Peel and cut the onions into eighths. (If griddling, separate the onions into ‘petals’.) Put a griddle pan on a high flame. Brush the vegetables all over with oil, and dunk the broccoli in oil, so the florets are coated. When the pan is very hot, lay in the courgettes in a single layer and grill for 2 minutes on each side, until pleasingly striped, then transfer to a platter.
Grill the onions for 5 minutes, until soft and blackened, then place on top of the courgettes. Grill the broccoli for 1½ to 3 minutes: you want to cook the stems without burning the florets, so use tongs to press them down, adding a splash of water to create some steam. Once tender, place on top of the onions.
Using tongs, hold the cob over a medium flame on the hob for about 5 minutes, rotating every 30 seconds or so, when the kernels start to blister and char. When the corn is cool enough to handle, stand it up in a bowl and cut down the length of the cob, close to the core, to shuck the kernels. Scatter these on top of the salad, whisk the dressing with a fork, pour over the top and gently toss to coat. Serve warm or at room temperature.