Place the sous vide immersion circulator in a large pot or container filled with water. Set the temperature to 150°F (65°C). Allow the water to come up to temperature.

Transfer the mashed potatoes to the resealable plastic bag. Remove as much air as possible from the bag. If you have a vacuum sealer, this is the best way to do it, but if not, you can use the water displacement method.

Once the water has reached the target temperature, submerge the bag of mashed potatoes in the water, ensuring the bag is fully submerged. Use a clip to attach the bag to the side of the pot if necessary to prevent it from moving or touching the heating element of the immersion circulator.

Allow the mashed potatoes to reheat for about 45 -90 minutes. The duration might vary depending on the quantity and thickness of your mashed potatoes. It's essential to ensure that the potatoes are heated through entirely, especially if they were cold from the fridge. You can check the internal temperature with a thermometer to ensure they're thoroughly heated.

Once reheated, carefully remove the bag from the water. Cut open the bag, and transfer the mashed potatoes to a serving bowl. If they've separated a bit, give them a good stir or whip to reintegrate any liquids and make them creamy again.

Taste and adjust the seasoning if necessary. You can also add a bit of butter or cream for extra creaminess.