Season the chicken well with salt and pepper on both sides. Add a tablespoon of oil to a large, deep frypan (28cm or above) and heat on medium high heat and fry the chicken for 4 minutes on the top side and then turn over and fry for 4 minutes on the other. Put aside.
Add a bit more oil and fry the onion until softened scraping up any bits left from frying the chicken from the bottom of the pan. Add the mushrooms and fry for 3-4 minutes. Then add the capsicum and garlic and fry until starting to become softened.
Add the red wine, tomatoes, tomato paste, olives, capers and chicken and place the lid on and bring to a simmer for 35-40 minutes. Add the chicken stock halfway through if the liquid dries out. Season with salt and pepper. Add 1-2 teaspoons of sugar to balance if the tomatoes aren't sweet. Serve with fresh herbs and pasta, bread or potatoes.