Heat a large frying pan over medium-high heat. Season duck breasts liberally with sea salt, place skin-side down in pan and cook until skin is golden and crisp (5-7 minutes), turn, add carrots and cook until duck is cooked through and carrots are golden (5-7 minutes). Remove duck and carrots, cover with foil and keep warm.
Add stock, Grand Marnier, vinegar and honey to pan, bring to the boil and cook until reduced by half (3-5 minutes), add orange slices, cook until heated through (1 minute) and season to taste. Serve duck and carrots with orange sauce and watercress sprigs.