1. Cut steak along the grain into 2-inch-thick strips, then cut each strip across the grain into 1/4-inch-thick slices.
2. In a medium bowl, combine steak, garlic, 1 tablespoon sherry, cornstarch, soy sauce and black pepper. Stir in 1 teaspoon oil.
3. Heat a large wok or skillet over high heat. Add 1 tablespoon oil, peppers and onion; stir-fry 1 to 2 minutes or until peppers begin to wilt. Transfer to a plate.
4. Add remaining oil, then steak in a single layer. Cook steak undisturbed 1 minute. Add chili powder; stir-fry 15 seconds or until just combined. Add pepper-and onion mixture, salt and remaining sherry; stir-fry 1 minute or until beef is just cooked through. Stir in cilantro.
5. Serve with warmed tortillas and top with yogurt or sour cream if desired.