Step 1

Heat oil in a 5 to 7 quart slow cooker using sauté function or in a large skillet over medium-high heat. Season sirloin with salt and pepper. Cook, turning occasionally, until browned on all sides, 4 to 5 minutes.

Step 2

Add onions and cook, stirring occasionally, until softened, 2 to 3 minutes. Sprinkle with flour and cook, stirring, 30 seconds. Add butter and mushrooms. Cook, stirring occasionally, until mushrooms just begin to steam, 1 to 2 minutes. If necessary, transfer ingredients to slow cooker. Stir in stock. Cook until beef is tender 2 to 3 hours on high and 4 to 6 hours on low.