Preheat oven to 160°C. Use a small knife to remove the base of each pear so they sit upright. Combine the wine, water, cassis, sugar and cinnamon in a saucepan over medium heat.
Cook, stirring, for 2-3 minutes or until sugar dissolves. Add pears and cook, turning occasionally, for 20 minutes or until tender. Set aside for 1 hour to cool.
Meanwhile, use a sharp knife to score the skin of each duck breast at 1cm intervals. Rub the duck with sea salt flakes and set aside for 15 minutes to develop flavours.
Heat a frying pan over medium-high heat. Add the duck, skin-side down, and cook for 5 minutes each side or until golden. Transfer to an ovenproof dish and cover with foil. Bake in oven for 10 minutes for medium or until cooked to your liking.
Use a slotted spoon to transfer the pears to a bowl, reserving 125ml (1/2 cup) of the poaching liquid. Place 2 teaspoons of liquid from the dish used to cook the duck in a saucepan. Add the stock, vinegar and reserved poaching liquid.
Bring to the boil over high heat. Boil for 3-5 minutes or until reduced by half. Add pears and cook for 5 minutes or until heated through.
Thickly slice the duck across the grain and divide among serving plates. Serve with the pears and sauce.