Heat oil in a large nonstick skillet. Add onion and garlic and crumble in turkey. Stir-fry over moderate heat until turkey is no longer pink- about 6 minutes. Mix in chili powder, salt, cumin coriander, tomato paste and water. Bring to a simmer and keep hot.

Divide lettuce among four dinner plates. Top with hot turkey mixture and sprinkle with cheese. Divide tomato wedges, avocado slices, if using, and corn chips among the four or six plates. Serve at once.