Start by placing the steak pieces in a large bowl and seasoning them with salt and pepper. Toss everything well so each piece is evenly coated. This simple step builds the base flavor for the dish.
Heat olive oil in a skillet over medium-high heat and sear the steak in batches. Make sure not to overcrowd the pan, as this helps create a nice brown crust. Searing locks in juices and adds a deep, rich flavor.
Once the steak is seared, transfer it into your slow cooker. Add sliced onions and garlic, spreading them evenly over the meat. These aromatics will infuse the dish with incredible flavor as it cooks.
Pour the beef stock over the steak, then sprinkle the au jus gravy mix on top. Place slices of butter over everything so it melts slowly into the mixture. This creates a rich, savory gravy.
Cover the slow cooker and cook on HIGH for 3–4 hours or LOW for 6–8 hours. Stir occasionally if possible, but even without stirring, the steak will turn out tender and flavorful.