Heavy bottomed ovenproof skillet A 6-inch cast iron pan
Preheat the oven to 400 degrees F.
With a sharp knife score the fat of the duck breasts in a criss-cross pattern. Season the duck with salt and pepper. Warm a heavy bottomed ovenproof skillet over medium heat.
Place the duck breasts, fat side down, in the skillet to render off the fat, about 6 minutes. Reserve rendered duck fat. Turn the duck breasts over and sear for 1 minute. Turn the fat side down again and place the skillet into the oven to roast for 7 to 9 minutes, until breasts are medium rare. Let the duck breasts rest for 5 minutes then thinly slice.
Grate the potatoes and squeeze out excess liquid using your hands. Toss the potatoes with the melted butter, rosemary, salt, and pepper. Heat 1 tablespoon of duck fat in a 6-inch cast iron pan over medium-high heat. Press some of the potato mixture into the hot pan to make a 1/4-inch thick cake. Reduce the heat to medium and cook until the rosti is golden brown, about 5 minutes. Flip the rosti in the pan and add more duck fat. Continue cooking until golden and crisp. Place the finished rosti onto an unlined baking sheet and continue cooking the rest of the potato mixture. Reheat rosti in a 400 degree F oven for 10 minutes.
Blueberry Green Peppercorn Chutney:
Combine the brown sugar, raspberry vinegar, red wine vinegar, and white wine vinegar in a medium pot. Place over medium-low heat and stir to dissolve the brown sugar. Simmer for 5 minutes.
Add the blueberries, onion, green peppercorns, lemon juice, and ginger. Reduce the heat to low and simmer for 45 minutes or until thickened, stirring occasionally to prevent scorching.
The chutney will keep for several weeks stored in the refrigerator in an airtight container.