Place the steak pieces into a large bowl. Season with salt and pepper and toss to combine to thoroughly coat
Get a large skillet hot over medium-high heat and add half of the oil. Brown (sear) the steak bites in batches. We're not cooking them all the way, just giving them some color. Add more oil to the pan if needed. Once all the steak is seared, add them into a 6-quart slow cooker along with the onion and garlic. Give it all a gentle stir
Pour in the beef stock. Sprinkle the au jus packet evenly over the top. Lay slices of the butter evenly over the top
Cover the slow cooker and cook on high for 3-4 hours or low 6-8 hours. Stir once an hour if you can. Serve with a garnish of fresh parsley