In a medium bowl, whisk together the beef broth, soy sauce, brown sugar, red pepper flakes, and garlic until well blended. Set aside.

In a large zipper bag, combine the sliced flank steak and cornstarch. Seal the bag and shake until the beef is coated.

In the slow cooker, add the coated flank steak, diced onion, and shredded carrots. Stir to mix the ingredients.

Pour the sauce over the mixture in the slow cooker. Stir well, cover, and cook on high for 2-3 hours or low for 4-5 hours.

Once cooked, serve the Mongolian beef over rice and garnish with sesame seeds and green onions.