In a small bowl, mix tomatoes, spinach, onion, Parmesan cheese, basil, oil, vinegar, ⅛ teaspoon pepper and garlic powder.
Sprinkle chicken with salt and remaining ¼ tsp. pepper; place on lightly oiled grill rack. Grill, covered, over medium heat 4-6 minutes on each side or until a thermometer reads 165°.
Top each chicken breast with 1 cheese slice; cover and grill 1-2 minutes longer or until cheese is melted. Grill tortillas over medium heat 20-30 seconds or until heated through.
Place chicken on center of each tortilla; top with about ¼ cup tomato mixture. Fold bottom of tortilla over filling; fold both sides to close.
