Slice the flank steak thinly against the grain, about 1/4 inch thick. Season with salt and pepper.

In a bowl, whisk together soy sauce, brown sugar, garlic, ginger, and beef broth until the sugar dissolves.

Place the sliced flank steak in the slow cooker. Pour the sauce mixture over, stirring gently to coat.

Set the slow cooker to low heat, cover, and cook for about 3 hours until the beef is tender.

Check for tenderness after 3 hours. If needed, extend cooking time by 30-60 minutes.

For a thicker sauce, combine cornstarch and water to form a slurry, add to slow cooker for the last 30 minutes.

Serve the Mongolian Beef over rice or noodles, garnishing with green onions.