01 -
Place sirloin steak pieces into a large bowl. Season evenly with kosher salt and black pepper, then toss to coat thoroughly.
02 -
Heat a large skillet over medium-high heat and add vegetable oil. Once hot, brown the steak bites in batches, searing only to develop color without cooking through. Add more oil as needed.
03 -
Transfer seared steak to a 6-quart slow cooker. Add thinly sliced onion and garlic. Gently stir to combine.
04 -
Pour unsalted beef stock over the mixture. Sprinkle au jus gravy mix evenly on top. Distribute thin slices of unsalted butter over the surface.
05 -
Cover and cook on high for 3-4 hours or on low for 6-8 hours, stirring once per hour if possible for even cooking.
06 -
Garnish optionally with fresh parsley and serve alongside mashed potatoes, rice, or egg noodles.