Heat the butter and oil in a large frying pan and brown the chicken thighs all over on both sides to seal (if using chicken breasts, cut each breast into 3 pieces). Remove the browned chicken pieces from the pan and put aside. Add the garlic and onion to the pan and cook gently for 4 minutes. Place the chicken pieces back into the pan and add the rosemary, lemon juice and zest and white wine and season well with salt and pepper. Turn down the heat and continue to cook the chicken for 20-25 minutes, turning occasionally. When the chicken is cooked it should be soft and tender with an aromatic smell of rosemary and lemon. Pour in the double cream and swirl the juices around in the pan to create a delicious creamy sauce. Serve the rosemary and lemon chicken on large plates and spoon over the creamy sauce. Sprinkle over chopped parsley and serve with oven roasted potatoes.