Preheat your oven to 180°C (350°F).

In a large bowl, vigorously whisk the eggs until well beaten.

Add the grated zucchinis, chopped spinach, parsley, and finely chopped onion to the beaten eggs. Stir the mixture until all the ingredients are evenly combined.

Season the egg and vegetable mixture with salt and pepper according to your taste preferences.

Grease an oven-safe skillet or baking dish with a little oil or butter, then pour in the frittata mixture, smoothing the top to ensure an even layer.

Place the skillet or dish in the preheated oven and bake for 20-25 minutes, or until the frittata is set in the center and golden brown on top.

Remove from the oven and let it cool for a few minutes before slicing into wedges and serving.