Heat the remaining oil in a heavy based or non stick frying pan. Pour in half the mixture, making sure it gets its share of the onion. Let it run in the pan but don’t expect it to fill the pan completely. Reduce the heat to moderate and let the frittata cook until almost set right through. Lift onto a plate then invert the frittata back into the pan uncooked side down and cook for 2 minutes more or until the underneath is slightly browned.