Preheat the oven to 180°C.
Truss the duck and place pork fat over the breast.
Roast the duck in a tin for 10 minutes.
Remove the fat and roast for 40 more minutes while basting.
Remove the bird from the oven and let it rest in a warm place.
Cook shallots and thyme in 25g of butter for 10 minutes.
Add vinegar for 30 seconds, then pour in port, wine, and cassis.
Reduce the liquid to a thin syrup and simmer with blackcurrant juice for 5 minutes.
Add blackcurrants to warm through and whisk in the remaining 25g of butter.
Carve the duck and serve with the sauce.