Place 3 pounds sirloin steak, cut into bite-sized pieces into a large bowl and season with 2 teaspoons kosher salt, 1 teaspoon black pepper and toss to combine.
Get a large skillet hot over medium-high heat and add 1 Tablespoon vegetable oil. Get it super hot then brown (sear) the steak pieces in batches. We're not cooking them all the way. Just browning them to give them some color. Add more oil to the pan if needed.
Once all the steak is seared, add them into a 6-quart slow cooker along with 1 small sweet onion, thinly sliced and 3 cloves garlic, thinly sliced. Gently stir together.
Pour in 1 cup unsalted beef stock. Sprinkle 1 ounce packet au jus gravy mix evenly over the top. Lay 4 tablespoons unsalted butter, thinly sliced over the top.
Cover the slow cooker and cook on high for 3-4 hours or low 6-8 hours. Stir once an hour if you can.
Then serve (with mashed potatoes, egg noodles or rice.)
Video
Notes
Flat iron steak is a great, less expensive alternative.
This is great served over mashed potatoes, rice or egg noodles.
You can cook this on low for 6-8 hours or high for 3-4 hours (cooking it slower will result in more tender steak bites.)
You do not need to brown the meat first before cooking (although this does add great color and flavor.)