Whisk flour, salt, pepper, and dry mustard together. Toss in steak and cover well with the flour.

Spray the slow cooker with cooking spray and add steak and flour.

Add the carrots, onions, and mushrooms. Add the beef broth and wine and stir to combine.

Cover and cook on low for 8 hours. You may want to stir this once or twice as it may stick a little.

Before serving add the sour cream and heat through. Serve over fluffy mashed potatoes.