Preheat the oven to 200°C. Wash mushrooms and cut into halves or divide into four the big one. Dice onion. Spice chicken breast with salt and pepper on both sides. Heat oil in a fry pan, sauté chicken on each side, remove and place in another pan. Transfer to the oven and roast for approx. 25 minutes, or until the chicken is thoroughly cooked through. Meanwhile add mushrooms in the frying pan, roast until golden-brown. Add chopped onion, spice with salt and pepper and fry for 3 minutes to soften. Deglaze then with port wine and add cream. Cook in a quick oven for few minutes uncovered. Stir in parsley and season to taste with salt and pepper. Serving with the chicken