Fruit-based sauces are often paired with game meats to temper their assertive flavor. Here fresh cherries are cooked with Cherry Balsamic Vinegar in a sumptuous, not-too-sweet sauce that complements the richness of the duck.
Demi-glace, a concentrated stock reduction, is added to the sauce to impart body and to mellow the fruit and vinegar combination. If demi-glace isn't available in your area, simply start with 1 cup of stock and slowly reduce it to 1/3 cup over low heat.
Combine the cherry juice, vinegar, and sugar in a small saucepan and bring to a boil over high heat. Add the cherries and cook until they soften but still hold their shape, about 5 minutes. Transfer the cherries to a bowl with a slotted spoon and set aside.
Return the pan to the heat and add the shallot to the liquid. Cook over medium heat until the liquid is reduced to 1/2 cup, 5 to 10 minutes. Add the stock and cook 1 minute. Set aside at room temperature while you cook the duck breasts. Alternatively, the sauce may be refrigerated at this stage for up to 2 days before finishing.
Season the duck breasts with salt and pepper. Heat a large skillet over medium heat until hot. Arrange the breasts in the pan, skin side down, so that the pieces don't touch. Cook until the duck skin turns deep golden brown, about 15 minutes, pouring off any fat as it accumulates. Turn the breasts over and cook until the duck is medium-rare to medium, about 5 minutes. Transfer the duck to a cutting board and let the meat rest for 5 minutes.
Pour off any fat from the skillet, add the balsamic cherry sauce, and bring to a boil. Remove the pan from the heat and whisk in the butter. Season to taste with salt and pepper.
Thinly slice the duck breasts on the diagonal and divide among 4 warm plates. Nap each serving with some of the hot cherry sauce and serve immediately.