Arrange duck legs in a single layer in a baking dish and season generously with salt and pepper. Cover and refrigerate overnight. (You can skip this and season them just before cooking, but if you have time it's worth it to season them a day in advance.)
Drizzle olive oil in the bottom of a slow cooker. Arrange duck legs over the oil in a single layer. Cook on low for 6 hours.
Transfer duck legs to a loaf pan and pour rendered fat from slow cooker over legs. Cover and refrigerate overnight.
When you are ready to eat, heat a heavy skillet (ideally cast iron) over medium-high heat. Place legs in the dry pan, skin side down, and cook until skin is golden brown and crispy and leg is heated through, 4 to 5 minutes. Carefully turn legs and let cook on opposite side for 1 to 2 minutes more. Serve.