Place 3 pounds of sirloin steak cut into 1-inch cubes into a large bowl. Season with 2 teaspoons of kosher salt and 1 teaspoon of freshly ground black pepper. Toss thoroughly to ensure all pieces are evenly coated.

Heat a large skillet over medium-high heat and add 1 tablespoon of vegetable oil. Allow the oil to get hot and shimmering. Add the steak pieces in batches to the skillet, being careful not to crowd the pan. Sear each batch to brown the exterior, adding more oil if necessary. You are only browning the steak for color and flavor, not cooking it through.

Transfer the seared steak pieces to a 6-quart slow cooker. Add 1 small sweet onion thinly sliced and 3 cloves of garlic thinly sliced. Gently stir the ingredients together to distribute evenly.

Pour 1 cup of unsalted beef stock into the slow cooker. Evenly sprinkle a 1-ounce packet of au jus gravy mix over the top. Finish by laying 4 tablespoons of unsalted butter cut into pats across the surface.

Cover the slow cooker and cook on high for 3-4 hours or on low for 6-8 hours. Stir once an hour if possible to ensure even cooking and proper flavor distribution.

Check for doneness after the minimum cooking time. The steak should be fork-tender but not falling apart. Serve the steak bites and sauce over mashed potatoes, egg noodles, or fluffy rice.