In a large pot, heat up the oil on a medium heat, add the chicken pieces, season them with salt and pepper, and brown on both sides.
Remove them from the heat, and set aside.
In the same pot, add the peeled and chopped onion, and fry until golden.
Add the peeled and chopped garlic, chopped red pepper, carrots, celery stalks, and mushrooms and let them cook for 3-4 minutes until the mushrooms release their liquid.
Return the chicken to the pot, add the olives, stock, chopped tomatoes and oregano, give everything a good mix, and place a lid on the pot.
Leave to cook until the chicken and veggies are cooked through, and the liquid is reduces into a sauce.
Check if it needs any more seasoning, and garnish with freshly chopped parsley.