Heat the oil in a large, nonstick frying pan with a flameproof handle, add the onion and courgettes and fry over gentle heat for 5 minutes or until lightly browned and just cooked.
Beat together the eggs, 2 tablespoons water, chopped mint and a little seasoning. Add the egg mixture to the frying pan. Cook, without stirring, for 4—5 minutes or until the frittata is almost set and the underside is golden brown.
Transfer the pan to a hot grill and cook for 3—4 minutes until the top is golden and the frittata is cooked through. Cut into wedges or squares and serve with a mixed salad, if liked.