1. Heat grill to medium.
2. Spread eggplant slices on 2 baking sheets or trays. Brush 1/4 cup oil over both sides of eggplant slices. Spray another baking sheet with cooking spray and arrange tomato slices on it. Season eggplant and tomatoes with pepper.
3. Oil the grill rack. Grill the eggplant slices until browned and tender, 5 to 7 minutes per side. Set aside. Place the baking sheet with the tomatoes directly on the grill rack; cover and grill until tomatoes are bubbling and warm, about 7 minutes.
4. Preheat oven to 350 degrees F. Spray six 7 to 10 ounce ramekins or custard cups (about the same diameter as an eggplant slice) with cooking spray.
5. Assemble the towers: Using a wide spatula, layer an eggplant slice, a tomato slice and a basil leaf in each ramekin. Sprinkle with about 1 tablespoon feta. Repeat the layering once more, then top with an eggplant slice. Press down lightly.
6. Place ramekins on a baking sheet and bake until heated through and juices are bubbling around the edges, 25 to 35 minutes.
7. To serve, run a paring knife around the sides of each ramekin. Invert a plate on top and, grasping ramekin and plate with oven mitts, carefully invert the ramekin onto the plate. (Use caution as both the ramekins and juices are extremely hot.) Remove the ramekins. The juices will form a sauce around the tower. Drizzle each tower with 1 teaspoon oil and garnish with a basil sprig. Serve hot, warm or at room temperature.