Finely grate the courgettes into a colander, sprinkle with salt, mix well and allow to drain for 30 minutes. Rinse them under running water and squeeze dry with your hands, then place them in a clean tea towel and squeeze again.
Transfer the courgettes to a bowl and add the egg, matzo meal, nutmeg and pepper to taste. Mix them together until well combined, then allow to stand for 20 minutes for the flavours to develop.
Meanwhile, to make the sauce, combine the lemon zest and juice, mint and yoghurt in a bowl, then cover and chill until ready to serve.
Heat a ridged, iron grill pan or nonstick frying pan over a medium heat. Place dessertspoons of the courgette mixture onto the pan, flattening each to form a thick cake. Dry-fry the cakes for 4-5 minutes on each side, until they are firm and browned. (You may need to cook them in two batches.) Garnish with the mint and serve hot, with the chilled sauce on the side.
Variation: if you cannot find matzo meal, use matzo crackers ground to fine crumbs in a food processor or crushed thoroughly with a rolling pin.