Combine water and cornstarch together in a saucepan to desired thickness. Add beef broth, soy sauce, rice vinegar, wine vinegar, red pepper flakes, sugar, garlic, and ginger. Bring to a boil, reduce heat, and let sauce simmer until thickened while you cook the vegetables, about 5 minutes.

Heat oil in a wok or large frying pan. Add asparagus, onion, and garlic; cook until slightly tender, about 5 minutes. Add boy choy and zucchini; cook until still slightly crispy, about 5 minutes more. Stir sauce into the vegetables and top with sesame seeds.