medium duck breasts
Kosher salt, freshly ground pepper
tablespoons Sherry vinegar or red wine vinegar
Frisée (for serving)
Score skin of duck breasts; season with salt and pepper. Place skin side down in a cold medium skillet; cook over low heat until fat is rendered and skin is dark golden, 12–15 minutes. Turn duck, increase heat to medium, and cook 2 minutes. Transfer to a plate.
Meanwhile, cut peel and white pith from oranges. Cut along sides of membranes to release segments into a medium bowl.
Pour off all but 2 Tbsp. fat from skillet. Add shallot, vinegar, and half of orange segments. Season with salt and simmer until thick, about 4 minutes. Thinly slice duck and serve over frisée with pan sauce and remaining segments.