Thinly slice the onion and the garlic. Chop the chilli, if used.
Heat the oil in a pan and soften the onion (for about 5 minutes).
Meanwhile, slice the mushrooms and peppers, then add these to the onion together with the garlic and chilli.
Simmer over low heat, under cover, for 10 minutes, then add the vinegar and tomato purée. Season with salt & pepper, cover and cook for about 20 minutes, until tender. After 10 minutes do check if it’s getting too dry - if it is, add a drop of water.
Stir in the parsley and serve.