There's just enough feta cheese in this wrap to showcase the classic Greek flavor, but not so much that it overpowers the other ingredients.
In a small bowl, mix together the yogurt, feta, onion, and lemon juice for the dressing.
For each wrap, heat a tortilla in the microwave for 10 seconds. Spread about 1/3 cup of the dressing on the tortilla (to within an inch of the edge). Next, layer 1/2 cup of chicken, several tomato slices, and 1/2 cup of salad greens over the dressing. Fold the sides of the tortilla toward the center, bring up the bottom to cover the filling, and roll it snugly.
Tightly cover each sandwich in plastic wrap and refrigerate for up to 6 hours. Cut in half at serving time. Makes 4 gyros.