Trim the end off the zucchini, peel the onion then coarsely grate both. Put them into a colander and sprinkle over some salt. Toss and leave while you prepare the rest.
If using fresh corn, peel and remove the kernels from the cob. If using frozen/can make sure it is defrosted and well drained.
Coarsely grate the mozzarella and grate the parmesan relatively finely.
Squeeze as much liquid as you can from the zucchini and onion and discard the liquid. Put the drained zucchini and onion in a bowl with the corn, mozzarella, parmesan, egg, flour, baking powder and pepper. Mix well so well combined and ingredients seem pretty evenly distributed, without breaking up the zucchini too much.
Warm a thin layer of olive oil in a medium skillet/frying pan over a medium heat (you can do more/less at once in a smaller/bigger skillet, as you find easier to manage). Put spoonfuls of the mixture into the skillet and press them down with a spatula/fish slice to gently flatten.
Leave the fritters to cook a couple minutes (around 3, but will depend a little on pan/heat) until you can see the underside starting to brown. Turn over and cook a couple more minutes on the other side. Remove from the pan and set aside, ideally covered, while you cook further batches.
Suggest serving with a little sour cream and paprika on top or just as they are.