Core, seed, and slice the peppers lengthwise. Slice the onion thinly. Slice the stoned black olives thinly.
Heat oil in a pan (with a cover). Toss sliced peppers and onion in oil. Season with salt and pepper. Cover and cook, stirring occasionally to prevent burning, for 10 to 15 minutes, until the peppers are soft but not mushy.
Two minutes before end of cooking time, add the sliced black olives to heat through.
Serve warm. This dish is better if allowed to cool a little.