Peel if preferred or leave skin on. Cut into uniform halves or half-inch thick slices — even sizing is critical since the water bath cooks at a constant rate. Pat dry, then place in a single layer in a zip-lock or vacuum-seal bag with minimal overlap. Remove as much air as possible; a vacuum sealer is best, but the water displacement method works with a zip-lock. Optionally add a drizzle of olive oil, salt, or fresh thyme if finishing by roasting later.

Set your sous vide circulator to 150°F. Once the bath is up to temp, lower the sealed bag in and make sure it's fully submerged — clip it to the container side if it floats. Cook 1 hour for slices or small pieces, 2–3 hours for whole or large sweet potatoes. At the minimum time, gently press the bag: the potato should feel tender and yielding. If still firm, return to the bath for 30 more minutes and check again.

Remove bag from water bath and open carefully — steam will escape. Potatoes are fully cooked and ready to season and serve as-is. For a roasted finish: preheat oven to 400°F, pat pieces dry, toss with olive oil, salt, and pepper, spread on a rimmed baking sheet, roast 30 minutes until bottom is browned, flip, roast another 15–20 minutes. The interior is already done — oven is for color and caramelization only. Pat dry before roasting or they'll steam instead of brown. Make-ahead option: chill sealed bag in fridge and roast the next day.