Preheat oven to 375°. Combine first 6 ingredients in a bowl.
Cut phyllo sheets in half crosswise. Working with 1 phyllo sheet half at a time (cover remaining dough to keep from drying), place phyllo sheet on a baking sheet. Coat phyllo sheet with cooking spray. Repeat with 2 more layers of phyllo. Sprinkle with 2 tablespoons cheese mixture. Repeat layers 5 times, ending with 2 phyllo sheets. Coat top phyllo sheet with cooking spray; sprinkle with 2 tablespoons cheese mixture. Pat tomato slices with a paper towel. Arrange tomato slices on top of cheese, leaving a 1-inch border. Sprinkle with onions and the remaining 6 tablespoons cheese mixture. Bake at 375° for 20 minutes or until golden.
Sprinkle with fresh basil leaves.