Crush all sea salt and spices together. Wipe duck with paper towel and pat dry, then rub spice mixture over duck. Let sit for 20 min at room temperature.
Put duck in a slow cooker. Add finely chopped onion and garlic, then pour butter over meat. Cover and cook on low for 6 hours.
Strain out cooking juices. Cool quickly so fat solidifies and broth separates. Remove fat with a spoon and save broth for sauce.