In a small bowl, stir together cumin, chili powder, onion powder, salt, and red pepper flakes. Set aside.
Heat a large skillet over high heat. Add in 2 tablespoons of olive oil, let heat for a minute, and then add in your sliced bell peppers, onion, and mushrooms. Saute until the vegetables are tender-crisp (7-10 minutes).
During the last two minutes of cooking add in half of your seasoning mix and the minced garlic. Cook an additional 1 to 2 minutes. Remove the vegetables from the pan and transfer them to a serving dish, but keep the skillet hot.
Return the skillet to high heat. Add the thinly sliced flank steak to the hot skillet and sprinkle in the remainder of the seasoning mixture, and Worcestershire sauce. Cook the steak, tossing constantly until the meat is browned about 2 to 3 minutes for medium-rare. Flank steak is best-eaten medium-rare. For well done, add an additional 3 to 4 minutes.
When the meat is cooked, add the vegetables back in. Drizzle in lime juice and toss together.
Serve on a warm tortilla. Serve plain or top with sour cream, cilantro, guacamole, or salsa.