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 behinds to brown, 4 to 5 minutes.

Stif the cornstarch mixture again and pour it into the wok. Stir in the beef, peanuts, red pepper flakes, and black pepper, and stir-fry until the sauce thickens and the beef is heated through 2 to 3 minutes. If necessary, reduce the heat to keep the sauce from evaporating.

Nutrition: 205 calories, 20 g protein, 10 g fat, 9 g carbs, 46 mg cholesterol, 241 mg sodium