Heat 1 tablespoon of the oil in a large, non-stick frying pan with a flameproof handle over a medium heat. Add the onions and stir for 3 minutes. Add the garlic then continue frying for a further 2–3 minutes, stirring occasionally, until the onion is soft and golden brown.
Meanwhile, whisk the eggs and milk together in a large bowl with the herbs. When the onions are golden, stir them into the egg mixture.
Heat another 2 tablespoons of the oil in the pan. Add the zucchini and gently fry for about 5 minutes, stirring frequently, until tender and lightly browned. Add the zucchini to the egg mixture.
Add the remaining oil to the pan and swirl it around to coat the edge. Pour in the eggs and zucchini, using a spatula to spread them out. Leave the eggah to cook for about 8 minutes, shaking the pan occasionally, until the egg mixture is almost set and only a small amount of unset mixture remains on the surface. Meanwhile, preheat the grill to high and position the grill shelf about 10 cm from the heat.
Place the pan under the grill and grill the eggah for 2–3 minutes until the surface is just set. Run the spatula around the edge and invert onto a large platter. Serve immediately or set aside and leave to cool. Serve cut into wedges, sprinkled with chilli sauce or harissa, if you like.