In a large skillet, heat the olive oil over medium high heat.

Add the beef, onion, and garlic and cook until the meat is browned on all sides, about 5 minutes. Transfer the browned beef to a 4- to 6-quart slow cooker.

Add the mushrooms, sea salt, and black pepper on top of the beef in the slow cooker.

Pour the beef broth over the meat and vegetables. Cover and cook on LOW for 4 hours.

Stir in the Dijon mustard, sour cream, and parsley until blended.

Serve in bowls and garnish with dollops of sour cream, if desired.