Place the 3 pounds of sirloin steak, cut into bite-sized pieces, into a large bowl. Season generously with 2 teaspoons of kosher salt and 1 teaspoon of black pepper. Toss everything to combine thoroughly.

Get a large skillet super hot over medium-high heat. Add 1 Tablespoon of vegetable oil. Once shimmering, brown the steak pieces in batches. You are not cooking them all the way through, just searing them to develop a nice color and deeper flavor. Add more oil to the pan if needed between batches.

Once all the steak is seared, carefully add the browned beef pieces into your 6-quart slow cooker. Add the 1 small sweet onion, thinly sliced, and 3 cloves of thinly sliced garlic. Gently stir these ingredients together with the steak.

Pour in 1 cup of unsalted beef stock over the steak and onion mixture. Evenly sprinkle the 1 ounce packet of au jus gravy mix over the top. This provides the base for our rich gravy.

Lay the 4 tablespoons of unsalted butter, thinly sliced, over the top of the ingredients in the slow cooker. The butter will melt and enrich the sauce during cooking.

Cover the slow cooker with its lid. Cook on the high setting for 3-4 hours, or on the low setting for 6-8 hours. If possible, give it a gentle stir once an hour to ensure even cooking and distribution of flavors.

Once the beef is fork-tender, stir the contents of the slow cooker to combine the rich gravy with the meat. Then, serve this delicious slow-cooked steak hot with your desired sides.