This method can be done with really any small game leg. Duck and goose is a natural because they often have a lot of fat under the skin. But you can do this with pheasant legs, grouse legs, turkey legs, or even rabbit and squirrel legs. Obviously you'll need some extra fat with the skinnier animals, though. (Here's how to render your own duck fat) Butter or lard are good substitutes if you can't get duck or goose fat. And yes, olive oil works, too.
Pat the duck or goose legs dry with paper towels. If you have store-bought duck legs, prick the skin of the duck all over with a needle or the point of a sharp knife. Do not pierce the meat itself. Piercing the skin gives the fat a place to seep out. Salt your duck legs well and set them aside, skin side up. Let them come to room temperature for at least 30 minutes and up to 90 minutes.
Put the legs in a small casserole. How small? You want the casserole to be just big enough to hold the legs. Now you need some fat. If the legs themselves are fatty, you will only need to pour a thin sheen of oil or melted duck fat on the bottom of the casserole, then place the duck legs close together, but not overlapping. If the legs are skinny, add enough fat to come about 1/4 inch up the sides of the dish.
Put the casserole in the oven and turn it to 300°F; if you have a digital oven, you could even go down to 285°F. Do not preheat the oven. Every duck has a different level of fat, so doneness is more an art than a science. But it will take at least 90 minutes, and probably two hours, and even 3 or 4 hours won't hurt them. After 90 minutes, check the duck: It should be partly submerged in melted fat and the skin should be getting crispy.
When the skin is starting to look crispy, turn the heat to 375°F. Check after 15 minutes. You’re looking for a light golden brown. Remove the casserole from the oven and let cool for 10 to 15 minutes before eating. Save the accumulated fat for cooking vegetables, other meats or for keeping your skin shiny. I strain the fat through a paper towel, but you really only need to do this if you are saving the fat for several weeks or months; strained, it will keep for 6 months tightly covered in the fridge. Well wrapped, the duck meat itself will last up to 2 weeks in the fridge.
Keys to Success
You need the meat to swim in fat. It won't work otherwise. If the notion of that much fat frightens you, you can use chicken or duck stock in a pinch. Not as good, but it will keep the legs from drying out.
Your duck legs will keep a week in the fridge, and freeze well.
To reheat, set them under a broiler or toasted oven to re-crisp the skin. The meat is perfectly good at room temperature, so you'll only really want to get that skin nice again. No need to overcook the legs on reheating.