Season the chicken breasts with the salt and pepper. Heat the oil in a large frying pan and saute the chicken pieces over a high heat very briefly to seal on both sides. Transfer the chicken to a plate.
Deglaze the pan with the sherry, then add the orange juice, chicken stock and orange zest. Bring to the boil, turn heat to low and return the chicken breasts and their juices to the pan.
Simmer, uncovered for 10 minutes, turning once, until the chicken is cooked through. Transfer the chicken to a serving plate and keep warm.
Bring the liquid to a rolling boil and add the mint. Reduce the sauce to about 6 fl oz and check the seasoning.
Pour the sauce over the chicken and serve garnished with the orange slices and mint sprigs.