Place the steak in a large bowl and toss with salt and pepper to combine.
Heat a large skillet over medium-high heat and add half of the oil.
Sear the steak pieces in batches in the hot skillet, transferring them to an 8-quart slow cooker.
Add more oil to the skillet if necessary.
Once all the steak is seared, add onions and garlic to the slow cooker, tossing them with the steak bites.
Pour in the beef stock and sprinkle the au jus packet over the top.
Place slices of butter over the mixture.
Cover the slow cooker and cook on high for 3-4 hours or on low for 6-8 hours, stirring every 30 minutes if possible.
Serve with a garnish of fresh parsley if desired.