Heat oil in heavy skillet over medium flame; add onion and cook until softened. Stir in chicken and cook until no longer pink in the center, about 7 minutes. Add garlic, cumin and chili powder and cook a minute longer. Stir in tomatoes and green chilies and bring to a boil. Reduce heat and simmer, uncovered, about 10 minutes. Add salt and pepper to taste and remove from heat. Serve with tortillas, cheese and shredded lettuce.