Pat steak dry with paper towels, then season all sides generously with salt and pepper. Add a thyme or rosemary sprig to the bag for extra flavor. Place steak flat in a vacuum-seal or heavy-duty zip-lock freezer bag. Zip-lock users: use the water displacement method — slowly lower the open bag into the water bath so pressure pushes out the air, then seal just before it submerges. Keep the cut no thicker than 2 inches for even cooking; flatten gently if needed. Have paper towels, a heavy skillet, and high-smoke-point oil ready for the sear.

Fill a pot with water and attach your immersion circulator. Set temperature for your desired doneness: 120–128°F for rare, 129–134°F for medium rare (130°F recommended). Let the water fully preheat before adding the bag. Submerge the sealed bag completely and clip it to the pot side so it stays submerged and water can circulate. Don't let the bag float or bunch. Cook times at 130°F: 1–2.5 hours for thinner cuts, up to 4 hours for thick or dense cuts. Tenderloin (~2 inches): 45 min–4 hours. Fattier cuts like ribeye do better toward the longer end. Don't exceed the upper time limit — texture can turn too soft. No visual cues to check. Trust the temperature and timing.

Rest the steak in its sealed bag for 10–20 minutes, then remove and pat completely dry with paper towels — any surface moisture will steam instead of sear and kill your crust. Heat a cast iron or heavy skillet over high heat until very hot. Add a high-smoke-point oil and sear about 1 minute per side until a deep brown crust forms. Work fast — the inside is already done. Rest on a paper towel-lined plate, loosely tented with foil, for 1–5 minutes. Serve immediately, optionally with compound butter.