Preheat the oven to 350C/180C. Grate the zucchinis on the large side of a box grater. Finely dice the scallions, only using the green tips, and discard the white ends. Roughly chop the spinach if the leaves are large. Roughly chop the mint leaves, dill, and parsley. Transfer to a large bowl and crumble over the feta cheese. Add a good grind of black pepper.
Add the eggs to the mixture. Use your hands or a wooden spoon to thoroughly combine and coat all the leaves with the egg mixture. If using lemon zest, grate it in now.
Heat the oil in a large, ovenproof skillet over medium heat. Pour in the frittata mixture and cook for 5 minutes, just enough for the base to form a layer. Settle the top layer, making sure it’s even but take care not to disturb the base. Remove from the heat.
Transfer to the center rack of the oven. Cook for 25-30 minutes or until the center of the frittata is set. Remove from the oven and leave to stand for 15 minutes before serving. Cut into wedges. Enjoy hot or cold from the fridge.