For the Marinade: Combine all of the ingredients, except cornstarch and water, in a saucepan and bring to a boil. Whisk together cornstarch and water in a small bowl to make a slurry. Add the slurry to the mixture and stir well. Bring back to boil and cook for 5 minutes. Let cool.
Combine the beef and the marinade and refrigerate for 24 hours.
Preheat a grill.
Skewer beef cubes, red bell peppers, red onion and cherry tomatoes. Skewer each item separately as cooking times vary. Barbecue over low heat, turning and basting with marinade for 20 to 30 minutes or until done.