Place pitas on a baking sheet. Spread 1/2 teaspoon garlic on each pita. Sprinkle each pita with 1/4 cup cheese, and divide onion evenly among pitas. Bake at 475° for 8 minutes or until edges are lightly browned and cheese is bubbly.
While pitas bake, combine vinegar, oil, and pepper in a large bowl, stirring with a whisk. Stir in tomatoes, olives, and basil. Add salad greens, and toss gently to coat.
Place 1 pita on each of 4 plates; top each pita with about 1 cup salad. Serve immediately.