You need a heavy-bottomed frying pan. If it is large, make two frittate. If it is smaller, make four.
If you have no leftover risotto, cook the rice in double the quantity of boiling salted water for 10 minutes and drain. Stir all the other ingredients except for the butter into the cooked rice or risotto.
For each frittata, melt 1/8 stick of butter in the pan. Put in enough rice mixture to make a flat, even pizza-like round. Cook on a very low heat, turning the pan round slowly clockwise so that it cooks evenly. When the bottom has turned crisp and golden, cover the pan with a large plate, invert it to turn out the frittata, add more butter to the pan, and cook the other side the same way. Don't get tempted to hurry the procedure. The end result will be worth your patience.
Recipe courtesy of Diane Seed's Top 100 Italian Rice Dishes (10 Speed Press, 2001)