Start by cutting up the peppers into fairly thin length-wise strips, about 1 cm (1/4-inch) wide, following the directions in our post on "how to slice a bell pepper". Peel and slice the onion lengthwise as well into strips of the same size.

Heat a generous amount of olive oil in a sauté pan and add the onion. Sauté the onion over a moderate flame until it is just beginning to wilt, then add the peppers all in one go.

Without try to mix, cover the pan and lower the heat. Let the peppers and onions simmer gently for a few minutes until the peppers have reduced by a good quarter and softened considerably. Uncover and raise the heat again, not too high, just enough for a moderate sauté.

Mix the onion and peppers together, and sauté over moderate flame until the peppers are quite tender (but not mushy) and ever so slightly browned around the edges. If they start to brown more than you want before they're done, add a splash of water or white wine vinegar along the way.

When the peppers are just about done, season them with salt and pepper and, if using, mix in the capers and/or olives, then let the peppers sauté for another minute or two.

When the peppers are done to your liking, take them off the heat, cover them, and allow them to cool completely. They taste best when made several hours ahead. When you are ready to eat the peppers, just reheat them gently and serve. They are also quite good served at room temperature.