Place 3 pounds of sirloin steak, cut into bite-sized pieces, in a large bowl. Season with 2 teaspoons of kosher salt and 1 teaspoon of black pepper, tossing to coat evenly.
Heat a large skillet over medium-high heat and add 1 tablespoon of vegetable oil. Once the oil is very hot, sear the steak pieces in batches, browning them on all sides to develop color but not cooking them through. Add more oil to the skillet if needed between batches.
Transfer the seared steak to a 6-quart slow cooker. Add 1 small sweet onion, thinly sliced, and 3 cloves of garlic, thinly sliced. Stir gently to combine.
Pour 1 cup of unsalted beef stock over the steak mixture. Evenly sprinkle a 1-ounce packet of au jus gravy mix over the top. Lay 4 tablespoons of unsalted butter, thinly sliced, on top of the mixture.
Cover the slow cooker and cook on high for 3-4 hours or on low for 6-8 hours. If possible, stir the mixture once every hour to ensure even cooking.
Serve the tender beef bites with your choice of mashed potatoes, egg noodles, or rice for a complete and satisfying meal.