Thinly slice the flank steak against the grain. Season with salt and pepper and place in a large, zip-top bag. Add 1/4 cup cornstarch to the bag and shake to coat well.
In a large skillet, heat 1-2 tablespoons vegetable oil over medium-high heat. Remove the steak from the cornstarch mixture (discard extra cornstarch) and brown for about 3 minutes, stirring often. Add meat to a lightly seasoned slow cooker.
In a medium bowl, stir together 2 tablespoons sesame oil, 1/2 cups soy sauce, 2 cloves garlic, 1/2 cup beef broth, 1/2 cup brown sugar, 1/4 cup diced onion, 1 teaspoon grated fresh ginger and 1/4 teaspoon red pepper flakes. Pour over meat in slow cooker and cook for 4-5 hours on low.
Serve over rice, with sesame seeds and sliced green onions for garnish.