Clean the red and yellow peppers, discarding seeds and stems. Slice peppers into rings and set aside. Slice onion thinly and set aside.

In a nonstick skillet over medium-low heat, warm a small amount of olive oil. Add sliced onions and saute until tender, about 8 minutes. Using a fork, transfer the onions to a bowl and set aside.

Warm the pan again over medium heat. Add sliced bell peppers and saute gently until soft (15-20 minutes), tossing frequently to cook them evenly. Add them to the onions and let both cool in the bowl. (Adding hot peppers to the egg mixture would cook it too fast.)

In another bowl, beat the eggs, then add the cooled onions and peppers. Stir in parsley, Parmesan cheese, and salt and pepper to taste.

In the same saute pan, heat the remaining olive oil over medium heat. Add the egg mixture. Reduce the heat to low and cook until the frittata sets but is not browned, about 12-15 minutes.

To finish cooking, place the skillet under a preheated broiler, six inches from the heating element. Broil until the top is golden, about 1-2 minutes. Using a spatula, loosen the edges and slide the frittata onto a serving dish. Cut into wedges and serve warm or at room temperature.