Toss duck legs with salt, thyme, sage, rosemary, garlic, shallot, peppercorns, coriander seeds, bay leaves, and juniper berries. Arrange in a single layer in a glass or ceramic dish, cover, and refrigerate for 12 to 24 hours.

Brush off the cure and rinse duck legs under cold water. Pat completely dry with paper towels.

Preheat oven to 210°F (100°C). Arrange duck legs snugly in a single layer in a metal roasting pan. Pour duck fat over the legs to fully cover them.

Heat the pan on the stovetop until the fat begins bubbling across the surface. Cover tightly with parchment paper, then a double layer of aluminum foil. Transfer to the oven and roast for 8 hours until the meat is fall-apart tender.

Remove duck from the fat and pat off excess. Preheat oven to 465°F (240°C). Pour boiling water into a roasting pan and set a wire rack over it. Place duck legs skin-side up on the rack.

Roast for 40 minutes at high heat until the skin is deep golden and shatteringly crispy. Rest briefly, then serve with lentils or crispy potatoes.