Season the sirloin steak bites with kosher salt and black pepper.
Heat vegetable oil in a skillet over medium-high heat and sear steak bites until browned on all sides, about 2-3 minutes.
Transfer the seared steak bites to the slow cooker.
Add the thinly sliced sweet onion and garlic over the steak.
Pour in the unsalted beef stock and sprinkle the au jus gravy mix evenly.
Top with sliced unsalted butter.
Cover and cook on low for 4 hours until the steak is tender and flavorful.
Serve hot, spooning the garlic butter sauce over the steak bites.