Remove the skin from the chicken thighs. Peel the onions and garlic, cut the onions into strips and finely chop the garlic. Quarter the mushrooms. Heat the oil in a large saucepan and brown the chicken pieces on all sides, season with salt and pepper, then remove from the pan. Brown the mushrooms too, season and remove from the pan as well. Gently fry the onion and garlic, stir in the tomato purée, sprinkle with sugar, fry briefly and deglaze with the wine. Allow to reduce briefly, then add the tomatoes and olives. Cover and simmer for 5 minutes. Return the chicken to the pan and simmer for 40 minutes. Cut the peppers into bite-sized pieces and add to the pan with the mushrooms, then cook for a further 10 minutes. Season to taste with salt and pepper. Serve and garnish with chopped parsley.