Grate the courgette into a sieve, add some salt and rest it over the sink. After 10 minutes, squeeze out the liquid.
Meanwhile, whisk the eggs, 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 add the courgette, oregano and minced garlic. Stir-fry for a minute or so, then pour over the egg, spreading the mixture out evenly. Scatter the feta cheese on top.
Put a lid on and cook for 3 minutes, then turn down the heat and cook until the egg is almost set on the surface.
Turn on the grill setting in the oven and 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 with mint leaves scattered on top.