Place the grated zucchini in a colander and sprinkle with salt. Mix and leave to drain for about an hour. Drain well by squeezing in a towel to remove all the water (do this in 3-4 batches).
Place the drained zucchini in a large bowl and mix with the onion, spring onions, parsley, dill, mint, flour, feta cheese, and half of the kefalotyri cheese. Add the beaten eggs with the pepper and oregano. Mix well.
Take a rectangular baking dish, approximately 23 x 33 cm, and grease the bottom with a little olive oil. Sprinkle with half the panko breadcrumbs. Pour in the mixture and spread evenly. Sprinkle with the remaining grated kefalotyri cheese and panko breadcrumbs.
Bake in a fan oven at 180˚ C for about 35-45 minutes until set and golden brown.
For the yogurt sauce, mix all the ingredients in a bowl. Serve with the zucchini pie after cutting it into squares.