Heat the butter in a casserole. Put the chicken and quickly turn them on both sides.
Place the casserole in the oven for 5 to 6 minutes. Remove the breasts.
Prepare the sauce. Pour the stock and the Port into the casserole. Boil over high heat for 2 minutes. Stir in the cream and boil again at high heat for 1 or 2 more minutes until sauce has thickened. Add salt, pepper and lemon juice if needed.