Fill a large pot or container with warm water and heat the water bath to 145° F / 62.7° C according to your sous vide machine's instructions.
Drizzle a little olive oil on top of the chicken breasts, then liberally season on both sides with kosher salt and freshly ground pepper. Place a thyme sprig on each side of the breasts, then place the chicken in a vacuum seal bag or Ziplock bag.
Vacuum seal the bag if using a vacuum sealer; or if using a Ziplock bag, use the water displacement method: Partially seal it, removing as much air as possible, then place it in the water to push the rest of the air out before sealing the rest of the way.
Clip the bag to the side of the pot or container to keep it submerged, and cook for 1 ½ to 2 hours (3 if the chicken is frozen).
Remove from the water bath and rest for 5 to 10 minutes in the bag.
Remove the chicken from the bag and place on a cutting board to slice or dice.
To sear skin-on breasts: Heat a heavy-bottom skillet over high heat with a little canola oil until it just starts to smoke. Dry the skin with paper towels, season again with a little more kosher salt and pepper. Sear the chicken breast skin-side down for 45 seconds to 1 minute to brown and crisp up the skin. Serve immediately.
This recipe is for two chicken breasts but works the same for more or less quantity. Just keep them flat in the bag.
If you want to make shredded chicken, cook at 160° F / 71º C; you'll get a texture that's slightly firmer and a little stringy.