Place a heat-resistant pad underneath a large pot filled with water. Attach a sous vide precision cooker to the pot and set temperature to 195 degrees F (95 degrees C). Set timer to 30 minutes.
Cut the ends off the parsnips and peel. Cut a 2-inch piece off the smallest part of the parsnips and remaining parsnips into equal-sized pieces. Place into a resealable bag, making sure that all pieces are laying flat and are not overlapping. Add salt and olive oil.
Remove all air from the bag by using the water immersion method or a vacuum sealer. Lower the bag into the pot once the temperature has reached 195 degrees F (95 degrees C).
When the timer on the sous vide reads 10 minutes, melt butter in a small skillet over low heat. Add thyme sprigs and simmer for 10 minutes.
Remove bag from the pot and take out parsnips. Remove thyme from skillet and season butter with nutmeg. Turn up heat to medium-high and add parsnips. Cook until parsnips are lightly browned, 2 to 3 minutes.
