Set up an immersion circulator in a large pot and preheat the water bath to 155°F.
Season duck all over with salt, pepper and orange zest. Place garlic under the meaty side of each leg and add thyme sprig. Place duck lugs into vacuum bags and seal according to instructions.
Add sealed duck to water bath and cook for 36 hours. You will need to top off the water as it will evaporate. Add a wet kitchen towel on top to keep the bag submerged.
Remove the duck from water bath and transfer to refrigerator to chill. You can keep in the fridge for up to a week.
When ready to eat, remove to duck from vacuum bag. Scrape away the thyme sprig and extra fat. Heat skillet over medium-high heat (no oil needed) and add duck, skin-side down. Cook for 5 minutes before flipping and cooking for another 2 minutes.
If you don't have a vacuum sealer, you can use a freezer grade zipper bag and zip almost to closed. Submerge the bag in the water and the air will start to come out and zip once everything is released.
Place some wet towels on top of the duck so it remains submerged.
Add some water every few hours as it will evaporate.
You can also get crispy skin by broiling for 5-7 minutes on a baking sheet until crispy.