Season the Beef: Place 3 pounds sirloin steak pieces in a large bowl. Sprinkle with 1 teaspoon kosher salt and 3/4 teaspoon fresh ground black pepper and stir to coat.
Sear the Steak: Heat 1 tablespoon extra virgin olive oil and 1 tablespoon unsalted butter in a large cast iron or non-stick skillet over medium-high heat. Add steak in batches, browning on all sides (about 2–3 minutes per batch). Do not overcrowd the pan — work in batches for best color. Steak does not need to cook through. Add another pat of butter or drizzle of oil as needed if the pan gets dry. Transfer seared steak to a plate and set aside.
Prepare the Gravy Base: In a small bowl, whisk together 1 cup beef stock, 1 packet dry Au Jus gravy mix, and 3 garlic clovess until well combined.
Layer in the Crockpot: Spread 1 medium sweet onion sliced over the bottom of a 5-quart (or larger) crockpot. Add seared steak bites on top of the onions. Pour gravy mixture over the beef. Scatter the remaining 4 tablespoons unsalted butter over the top.
Slow Cook: Cover and cook on high for 3–4 hours, or on low for 6–8 hours, until beef is tender. Stir occasionally if possible.
Thicken the Gravy: During the last 30 minutes of cooking, whisk together 2 tablespoons cornstarch and 3 tablespoons water in a small bowl to form a slurry. Stir into the crockpot. Cover and continue cooking for 30 minutes until the gravy thickens into a silky sauce.
Serve and Enjoy: Stir beef and gravy gently before serving. Enjoy over mashed potatoes, rice, or noodles.
Notes
Make-Ahead Tips: You can season and sear the beef a day ahead, then refrigerate until ready to load into the crockpot. The sauce base can also be pre-mixed for a true dump-and-go morning.Storage: Store leftovers in an airtight container in the refrigerator for up to 3 days.Freezing: Place cooled steak bites and gravy in a freezer-safe container or bag. Freeze for up to 3 months. Thaw overnight in the fridge before reheating.Reheating: Reheat gently on the stovetop over medium-low heat or in the microwave. Add a splash of stock or water if the gravy has thickened too much.