Pat the beef sirloin dry with paper towels. Cut the meat into steaks, then into strips about 2 1/2 inches long by 1/2 inch thick. In a small bowl, mix the cumin, oregano, salt, and pepper, and season the beef strips. Let them marinate while you prepare the other ingredients.

Heat a wok over high heat with a little vegetable oil until it starts to smoke. When the wok is smoking, sear the beef on all sides. Cook only half the beef at a time, then the other half, so the wok stays hot. Set the browned beef aside on a plate. Add a splash of water to the juices left in the wok, stir, and set aside.

Heat the wok again until it is smoking. Stir-fry the onion and garlic for a few seconds, then add the rest of the vegetables and cook until just tender-crisp.

Add the reserved juices and soy sauce to the vegetables. Return the beef to the wok, mix well, heat through, and serve. You can serve this with rice or French fries.