Score the skin of the duck breasts.

Season the duck breasts with kosher salt and freshly ground pepper.

Place the duck breasts skin side down in a cold medium skillet.

Cook over low heat until the fat is rendered and the skin is dark golden, about 12-15 minutes.

Turn the duck breasts over.

Increase the heat to medium.

Cook for 2 minutes.

Transfer the duck breasts to a plate and let rest.

Cut the peel and white pith from the blood oranges.

Cut along the sides of the membranes to release the orange segments into a medium bowl.

Pour off all but 2 tablespoons of fat from the skillet.

Add the chopped shallot, Sherry vinegar (or red wine vinegar), and half of the orange segments to the skillet.

Season with salt.

Simmer until the sauce is thick, about 4 minutes.

Thinly slice the duck breasts.

Serve the sliced duck over frisee with the pan sauce and remaining orange segments.