Place the steak in a bowl and toss with the salt until evenly coated.
Heat enough oil to coat a large skillet (cast iron works great for this) over high heat. When the oil is shimmering, add the beef, spreading it evenly over the pan. You may need to do this in two batches. Sear for 1-2 minutes, or until browned, then flip and sear for an additional 1-2 minutes, or until browned but not cooked through. Transfer the steak to the slow cooker and repeat with the second batch.
Add the wine to the pan, scraping up any browned bits as it bubbles. Pour all the browned bits and juices from the pan into the slow cooker.
Stir in the beef broth, Worcestershire, garlic, and thyme. Whisk together the cornstarch and water to make a slurry, then stir it into the slow cooker as well. Top with the thyme and sliced butter.
Cook on low for 5-6 hours or on high for 2-3 hours, stirring once if possible. Before serving, remove any thyme stems. See notes for instructions to make a thicker gravy from the pan juices.