Start by grating the potatoes and zucchinis (keep about 100g each for the slices for topping). Place them in a cheesecloth or clean kitchen towel and add a pinch of salt and let sit for 10 minutes, then squeeze them to remove the excess water. Try and get as much water out as possible, this will ensure it stay crispy and that the vegetables don't release too much water while cooking. If you have some time, sprinkle the vegetables with salt, place them in a colander and cover them with a small plate and something heavy on top. The salt and weight will helps get more water out.
Preheat the oven to 200C/400 F (fan)Add the grated vegetables to a bowl, add sliced onion, season with dry herbs, salt and pepper and add plain flour and corn meal. Mix to combine, it will remain dry, then slowly add water, the mixture should be thick. If it's too watery (it also depends in how much water is left in the vegetables), add more flour. If it's too liquid, it will remain soggy. The ideal texture is like a wet bread dough.
Place baking paper on a baking tray, add some olive oil and spread it all over the paper.
Spread the mixture very thinly on it, then cover with courgette and potato slices, drizzle with olive oil, corn meal, salt.
Bake until crispy, for 40-50 minutes. To get the crunchy bottom, move the tray at the bottom of the oven for the last 10 minutes. If the top is getting burnt while the center is still not ready, cover it with some baking paper. Let it cool down a bit before slicing.