Gently stew the bell peppers and onion in some olive oil until they are quite soft. This will take anywhere between 20 and 30 minutes (or even more, if you want them very soft). Regulate the heat so that the vegetables only slightly brown, if at all. Season with salt and pepper, and allow them to cool, covered, for a few minutes. (This rest under cover will further soften the vegetables and, for some mysterious reason, tends to enhance their flavor.)

Then mix the sautéed vegetables with the eggs, which you will have beaten with some grated cheese and a pinch of salt and a grind or two of black pepper, and proceed as for any frittata.