Begin by cutting the flank steak into thin strips, approximately ½ inch wide. Place the beef strips into a ziplock bag, sprinkle with cornstarch, and shake until evenly coated. Set aside.

In your slow cooker, combine 2 tablespoons of sesame oil with 4 cloves of minced garlic, ½ cup of soy sauce, 1 cup of beef broth, ¼ cup of brown sugar, and 1 chopped onion. Add a pinch of red pepper flakes for heat if desired. Stir well.

Add the coated flank steak to the slow cooker and gently stir until every piece is thoroughly coated in the sauce. Spread the beef out evenly in the sauce.

Set your slow cooker to high for 2 to 3 hours or low for 4 to 5 hours. Do not lift the lid during cooking.

Once finished, check that the beef is fork-tender. Serve over rice or noodles and garnish with sesame seeds and sliced green onions.