Heat 2 teaspoons of the oil in a large grillproof nonstick frying pan over moderate heat. Add the onion and garlic and cook for 2 minutes. Add the greens and cook, stirring often, for 7 minutes or until very tender. Transfer the greens to a plate and wipe out the frying pan.
Combine the whole eggs, egg whites, ricotta cheese, Parmesan, flour and a pinch of salt in a food processor. Add the remaining oil to the frying pan and heat over moderate heat. Add the greens and the egg mixture, then cook without stirring for 10 minutes or until the edges of the frittata are set and the centre is slightly wobbly.
Preheat the griller. Grill the frittata 15 cm from the heat for 1 minute or just until the centre is set. Serve immediately.