Season steak pieces with kosher salt and black pepper. Toss to coat.
Heat oil in a skillet over medium-high heat and sear steak in batches until browned.
Transfer browned steak to slow cooker.
Sauté sliced onion and garlic in same skillet 2-3 minutes. Add to slow cooker.
Pour beef stock into slow cooker and sprinkle au jus gravy mix over contents.
Top everything with the slices of unsalted butter.
Cover and cook on low for 4–5 hours or high 2–3 hours until tender.
Stir before serving. Adjust seasoning with additional salt or pepper if needed.
Notes
Serve over mashed potatoes, rice, or egg noodles for the full cozy experience. Store leftovers in an airtight container for 3-4 days, or freeze for later.