Mix together all ingredients except oil. Heat a large, non-stick skillet (about 25 cm), add oil, and when it is medium-hot, add the mixture, spreading it evenly in the skillet. Cook over medium heat for about 5 minutes, or until bottom of frittata has formed a golden crust. Invert frittata onto a plate, then slip it back into the skillet to brown the other side. Cook until a crust is formed. Cut into wedges and serve, with more cheese, if desired.