Coarsely grate the zucchini and place in a strainer or sieve over a large bowl. Stir in ⅓ teaspoon of salt and set aside for a minute then using your hand squeeze as much moisture out of the mixture as you can. Set aside and discard the zucchini water.
Peel and finely grate the potatoes into a fine-mesh sieve placed over a bowl (the potatoes will start releasing moisture as you grate them). Add ⅔ teaspoon of salt and stir the mixture thoroughly squashing with a spoon to get rid of as much moisture as possible.
Slowly pour the potato water out of the bowl being careful not to remove the thick, starchy mixture that will have accumulated at the bottom (you will need to add it into the batter).
To a large bowl add the potato mixture, zucchini, egg, grated onion, potato starch and pepper to taste. Stir thoroughly.
In a large non-stick pan heat up 1 tablespoon of oil and drop spoonfuls of the batter flattening gently until about 5 mm thick. Cook over a medium heat for about 3 minutes, turn over, add a little more oil and cook the fritters for 3 more minutes or until nicely browned.
Place the fritters on top of a plate lined with kitchen paper (for about 30 seconds) then serve.
Notes
Once you’ve prepared the potatoes use them immediately (they discolour quickly).
It is important to grate the potatoes finely (this will help thicken the batter). Zucchini is soft and should, on the other hand, be coarsely grated (otherwise it would get lost in the potato mixture).
Stir the batter thoroughly. Also give a stir before making each batch (if you start seeing moisture in the batter just stir it in and continue making the fritters).
You do not need to add any salt into the batter.
I recommend using a non-stick pan.
Fry the fritters in hot oil (they will absorb less of it). Do not overcrowd the pan.
Best served within 1-3 minutes. After a while the fritters will lose their crispiness.