Set the sous vide to your desired temperature (130°F for medium-rare).
Fold over the top of the plastic bag and make a wide cuff on the vacuum-sealing bag. This will allow you to insert the steaks and keep the sealing area free from moisture and allow for a good seal.
When the water has reached the desired temperature, season the steaks well and place them in the bag. Add 2 or 3 fresh thyme sprigs. Unfold the cuff of the bag. Vacuum seal the bag with a food sealer and place in the water bath.
Cook for 1 hour or up to four hours. *(See Notes)
Remove bag from water bath and transfer the steaks to a plate. Pat the steaks with paper towels to remove as much moisture as possible and lightly brush with oil.
Preheat a cast iron skillet over high heat. Add the oil and let it get hot. When the oil is hot, add the butter and the steaks. Sear for 1 minute on each side. While the filets are searing, tilt the pan and spoon butter over the steaks as they cook. Hold the steaks with tongs and sear the edges.
Remove the filet mignon steaks and serve immediately.
Notes
No vacuum sealer? Use the water displacement method with a freezer-safe zip-top bag.Food safety tip: If cooking below 130°F, limit cook time to 2½ hours max.Searing safety: The final sear will create some smoke. Turn on your stove vent and crack a window or door before you start.Choose the right oil: Since you’ll sear the steak quickly over high heat, use an oil with a high smoke point such as avocado oil. Other good options include peanut, soybean, or canola oil.
TEMPERATURE AND TIMING CHARTS FOR SOUS VIDE STEAK
Very Rare to Rare: 120°F to 128°F : 45 minutes to 2½ hours
Medium-rare: 129°F to 134°F : 45 minutes to 4 hours*
Medium: 135°F to 144°F : 45 minutes to 4 hours
Medium-well: 145°F to 155°F : 45 minutes to 3½ hours