In a large bowl, mix together the cornstarch and vegetable broth, then add the vinegar, sugar, ketchup, soy sauce, ginger and cayenne pepper until well mixed. Set aside. In a large skillet or wok, stir fry the garlic, onion, and carrots in the oil about 5 minutes, until carrots begin to soften. Add the pepper and mushrooms and stir fry for another 3-5 minutes. Add the vegetable broth mix to the veggies and cook until mixture thickens, about 5-7 minutes. Serve over rice and enjoy!