Rub the duck legs thoroughly with the coarse sea salt, then place them in a dish with the thyme sprigs, smashed garlic, and black peppercorns. Cover and refrigerate for at least 2 hours to cure and deeply season the meat.

Preheat your oven to 300°F (150°C). Rinse the salt and aromatics off the cured duck legs under cold running water, then pat them completely dry using paper towels.

Place the duck legs snuggly in a single layer inside a small, deep baking dish or cast-iron skillet. Tuck the rinsed thyme, garlic, and bay leaf around the legs for extra aromatics.

Pour the melted duck fat over the legs until they are almost entirely submerged. Cover the baking dish tightly with a double layer of aluminum foil to seal in the moisture.

Bake in the preheated oven for 2 hours, or until the meat is incredibly tender and easily pulls away from the bone when gently pierced with a fork.

Carefully transfer the duck legs from the hot fat to a cold skillet, skin-side down. Cook over medium-low heat for 5 to 8 minutes until the skin is deeply golden and shatteringly crispy, then serve immediately with a drizzle of the warm fat over roasted potatoes.