This is an untraditional take on the Peruvian classic. I do this so you have crispy skin and the sauce is brighter and more flavorful than it would be if it were long simmered. Feel free to sub in chicken legs or pheasant thighs here.
Preheat the oven to 325°F. Salt the duck well and set them, skin side up, in a casserole or baking dish that just about fits them. Pour in the 2 cups stock only to the point it comes up to the skin of the legs. You don't want them submerged. Pop everything in the oven and let this bake for at least 90 minutes, or until the legs are tender.
Meanwhile, make the sauce. In a smallish pot or deep-sided pan, heat the duck fat over medium-high heat. Add the chopped onion and sauté for a few minutes, until it is translucent but not browned. Add the garlic and cook another minute or two. Add all the remaining ingredients, stir well and bring to a simmer. Cook gently for about 15 minutes.
Move the sauce to a blender and puree. Pour back into a small pot and leave it alone for the moment.
When the legs are tender, bring the sauce back to a simmer and spread some on everyone's plate. Top with the legs and garnish with cilantro. Serve with rice and/or boiled potatoes or yuca.
NOTE: If for some reason you happen to have the herb huacatay, which is common in Peruvian cooking, use it instead of the oregano.