Peel and chop the zucchini into slices, salt them to taste and leave aside, while you prepare the rest of the products.
Heat the oil in a small tray and braise the slices of zucchini on both sides until golden, take them out of the tray. In the same oil, braise the onions until softened and remove tray from the heat.
Add the zucchini and the very well beaten eggs, stir. Finally, sprinkle with the feta cheese and diced butter evenly.
Put the tray in a preheated 356°F (180 °C) oven and bake until ready.