Thinly slice the spring onions.
Grate the courgette into a sieve, add some salt and rest it over a pan. After 10 minutes, squeeze out the liquid.
Meanwhile, whisk the eggs with the chopped parsley, spring onions and minced garlic, then season with salt and pepper. Add the courgette and combine well.
Heat the oil in an oven-proof frying pan over medium heat, then pour in the egg mixture, spreading it out evenly. Scatter the cheese on top, if used. Put a lid on and cook for about 5 minutes, then turn down the heat and cook until the egg is almost set on the surface.
Turn the oven setting to grill, then brown the top of the frittata for a couple of minutes.
Let the frittata cool and cut it into wedges or squares.
Serve warm or at room temperature.