Whisk together beef broth, pear, garlic, soy sauce, red pepper flakes, ginger, sesame oil and rice wine vinegar. Place beef in bottom of slow cooker and pour the mixture over it. Cook on low for 5 to 6 hours, or high for 2 to 3 hours.
Whisk together cornstarch and water. Remove the meat from the slow cooker then whisk in cornstarch/water mixture to thicken sauce. Return meat and add scallions. Serve over brown rice!