Heat olive oil in a large skillet over medium-high heat. Sear steak bites for 2-3 minutes per side until browned. Transfer to the crockpot.
Top the steak with diced onion, minced garlic, au jus gravy mix, ranch seasoning mix, sliced butter, pepperoncini peppers with juice, and beef broth.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until tender.
Stir before serving.
Notes
Nutrition information was not provided on the source page and is not included. Stew meat, chuck roast, flank steak, or ribeye can substitute for sirloin. Halved baby potatoes and baby carrots can be added for a more filling one-pot meal. Leftovers keep refrigerated in a sealed container for 3-4 days.