Melt the butter in the 12" Large Skillet.
Cook the onion for about 5 minutes, stirring occasionally, until soft.
Remove the pan from the heat.
Sprinkle the ham, cheddar cheese, and Parmesan evenly over the onions.
Whisk together the eggs, egg whites, and evaporated milk.
Whisk together the flour, salt, baking powder, mustard, and cayenne pepper in another bowl.
Whisk the flour mixture into the egg mixture, then gently pour it over the onion mixture.
Cover the pan and cook over low heat for about 30 minutes until the edges are set and the center jiggles slightly when the pan is gently shaken.
Remove from the heat and let stand for 10 to 15 minutes until completely set.
Serve wedges warm or at room temperature.