Mix together the salt, pepper, ancho chile, coriander, cumin, and dried mustard.
Rub into the skin and flesh of the duck breasts.
Cover and let sit for 30 minutes to let the duck absorb the spices.
Preheat oven to 350°F/180°C.
Heat a large skillet over high heat. When hot, place duck breast into the dry skillet skin-side down. They will pop and sizzle.
Sear for 5 minutes, until skin is crispy and golden brown.
Turn and cook 30 seconds.
Place in preheated oven for 5 minutes (for medium-rare).
Transfer to a cutting board and slice.
Fan slices on the plate and drizzle with the warm sauce.