In a small bowl, combine the beef broth, soy sauce, cornstarch, and mustard, and stir until the cornstarch is dissolved. Set aside.
cut the broccoli into bite-size pieces.
In a nonstick wok or large nonstick skillet, warm the oil over medium-high heat until hot but not smoking. Add the broccoli, cauliflower, onion, and garlic, and stir-fry until the vegetables are crisp-tender and the onion begins to brown, 4 to 5 minutes.