Set duck breasts down on a cutting board, skin side up and pat dry with paper towels. Using a sharp knife, gently score the skin crosswise in ½” (1.3cm) intervals, being careful not to cut into the meat. Rotate the breast and score again, to make a crosshatch pattern. This will help the fat to render as the duck cooks. Season the duck breasts on both sides with salt and pepper.
Place the duck, skin side down in a large sauté pan. Do not add any oil to the skillet! Plenty of fat will render off the duck as it cooks. Heat over medium heat and let the duck cook, untouched, until the skin is golden brown and crisp, about 5 minutes.
Turn the duck over and sear the other side for 5 minutes. Turn it once more, skin side down, and sear for an additional 2 minutes. When you remove it, the skin should be crisp but the middle spongy but firm, cooked to a medium rare. (If you are using an instant read thermometer, cook to an internal temperature of 140°F/ 60ºC).
Remove the duck from the pan and allow it to rest on a cutting board for 10 minutes.
While the duck cools, return the pan to heat, reserving 2 tablespoons (30g) of the duck fat in the pan. Save the remaining fat for another use. Add the shallots and sauté until tender, about 5 minutes.
Add the blueberries, wine, maple syrup and rosemary to the pan. Turn the heat down to low and cook until the blueberries burst and the sauce thickens, about 15 minutes. Once the sauce has thickened, stir in the vinegar.
Slice the duck into strips, cutting any large strips into bite size pieces if desired.
Spoon the sauce onto plates and top with slices of the breast.
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Notes
Save extra rendered duck fat for cooking veggies or roasting potatoes - it adds amazing flavor.