Cut each chicken breast across horizontally into two slices.Coat each piece lightly with the flour.
Heat the oil and 20g of the butter in a frying pan large enough to take the chicken pieces in a single layer. Add the chicken and fry gently for about 5 mins on each side until tender. Transfer to a warmed serving dish and keep warm.
Add the lemon juice and stock to the pan juices. Bring to a boil, stirring and boil for a minute to reduce slightly.
Add the remaining butter and the parsley and stir until blended. Serve the chicken breasts with the sauce poured over and garnished with the lemon slices.