Blend the Marinade: Combine the lemongrass, coconut milk, peanut oil, fish sauce, soy sauce, brown sugar, ground coriander, turmeric, lime zest and juice, garlic, scallions, and serrano chile in a blender or food processor. Blend until you achieve a smooth consistency.
Marinate the Duck: Pour the blended marinade into a large mixing bowl. Add the duck pieces, tossing them to ensure they are evenly coated. Cover the bowl and refrigerate the duck for at least 2 hours, or leave it overnight for enhanced flavor.
Prepare Skewers: If using wooden or bamboo skewers, submerge them in water and soak for 30 minutes before threading the duck. This prevents them from burning during cooking.
Heat the Grill or Broiler: Preheat a grill or broiler to medium-high heat. Once heated, lightly drizzle peanut oil over the skewers to prevent sticking.
Cook the Duck: Thread the marinated duck chunks onto the prepared skewers. Place the skewers on a baking sheet if broiling or directly on the grill. Cook the duck for 3 to 5 minutes on each side, until it is just cooked through but still slightly pink inside.
Finish and Serve: Once cooked, transfer the skewers to a serving plate. Squeeze fresh lime juice over the top for added brightness, and garnish with chopped scallions and fresh cilantro or basil leaves. Serve alongside the optional Spicy Coconut Cashew Dip for dipping.