To make the rosti, wrap the grated potato in a clean tea towel and squeeze out all the excess water until the potato is dry. Put the potato in a bowl and season well.
Divide into four portions and, using your hands, squeeze together to form four thin patties. Chill them on a plate in the fridge for 20 minutes (or until needed).
Preheat the oven to 200°C, fan 180°C, gas mark 6. Line a baking tray with baking parchment.
Put 1½ tablespoons of the olive oil in a frying pan and brown the potato patties on both sides, then transfer them to the baking tray and bake for 10 minutes.
Meanwhile, cook the duck breasts according to the pack instructions and allow to rest.
For the sauce, heat the remaining ½ tablespoon of olive oil in a frying pan, add the sliced shallots and cook for 5 minutes to soften. Add the frozen cherries and red wine, and simmer until reduced by half.
Add the chicken stock and simmer for 5 minutes more until reduced, then gradually whisk in the cold butter and red wine vinegar, and season to taste.
Slice the duck and serve on top of the rosti with the cherry sauce poured over.