Preheat your sous vide water bath to your desired temperature. For a medium-rare ribeye, 130°F (54°C) is ideal. Ensure the water level is appropriate for your immersion circulator.

Season the steak generously on all sides with salt and freshly ground black pepper. Place the seasoned steak in a vacuum-sealable bag along with the rosemary sprigs and smashed garlic cloves.

Vacuum seal the bag, ensuring it's airtight. If you don't have a vacuum sealer, you can use a high-quality zip-top bag and employ the water displacement method to remove as much air as possible.

Carefully place the sealed bag into the preheated water bath, ensuring the steak is fully submerged. Cook for a minimum of 1 hour for a 1.5-inch steak, or up to 4 hours for increased tenderness and flavor infusion.

Once the sous vide cooking time is complete, remove the bag from the water bath. Take the steak out of the bag and pat it thoroughly dry with paper towels. This is a crucial step for achieving a good sear.

Heat the olive oil and butter in a cast-iron skillet or heavy-bottomed pan over high heat until the butter is melted and the oil is shimmering. Carefully place the dried steak into the hot pan.

Sear the steak for 60-90 seconds per side, until a deep brown crust forms. You can also sear the edges for added texture.

Remove the steak from the pan and let it rest for 5-10 minutes before slicing and serving.