Preheat the water bath with sous vide. Set water temperature to 160°F.
Slice a quarter of a small onion into thin slices. Mince two cloves of fresh garlic.
Remove the boneless short rib from its packaging and pat it dry with a paper towel. Place it into a sealable bag along with sliced onions, minced garlic, thyme, salt, and pepper. Vacuum seal bag.
Place the sealed bag into the water bath at 160°F for 48 hours.
Once the cooking time has ended, remove the short rib from the bag and pat it dry with a paper towel.
Reverse-sear the rib on a hot grill over high heat. Cook for 2 minutes per side.