Remove the outer leaves from each Brussels sprout and discard any that are blemished. Continue to separate the leaves of the Brussels sprouts, using a small, sharp knife to cut away the core.
In a large saucepan over medium heat, pour the duck fat, add minced garlic, Thai chiles, salt and pepper. Toss Brussels sprouts in. Sauté for 2 minutes or until slightly browned. Remove from heat. Add lemon zest. Toss and serve.