Measure and rub the duck legs generously with salt, pepper, and crushed garlic, then let them marinate in the refrigerator for at least 1 hour or overnight.
Preheat the oven to 275°F (135°C) and melt the duck fat in a large ovenproof skillet or Dutch oven over low heat.
Pat the marinated duck legs dry with paper towels and place them skin-side down in the melted duck fat, ensuring they are fully submerged.
Add fresh thyme sprigs and crushed garlic to the fat surrounding the duck legs for flavor.
Transfer the skillet to the oven and cook slowly for about 2 hours until the meat easily pulls away from the bone and the skin is tender but not crispy.
Remove the skillet from the oven, carefully lift the duck legs out, and increase oven temperature to 425°F (220°C).
Place the duck legs skin-side up on a baking sheet, then bake for 15 minutes until the skin is crispy and golden brown, audible sizzling indicates crisping.
Remove from oven and serve immediately, showcasing the crispy skin and tender meat with a rich, golden appearance.
