Add the port to the skillet and simmer over high heat until reduced by half, about 3 minutes. Add the orange juice and cook until reduced by half, about 2 minutes. Stir in the finely chopped chicken liver, reduce the heat to low and simmer until just cooked, about 1 minute. Remove the skillet from the heat. Stir in the mustard, season the sauce with salt and pepper and keep warm.