Pre-heat grill over high heat and brush both sides of eggplant slices with olive oil using a pastry brush. Sprinkle with salt and pepper.
When grill is hot, lay eggplant slices in a single layer and grill for 3-4 minutes per side or until tender. Remove from grill and place in a shallow baking dish. Top each slice of eggplant with one or two tablespoons of marinara, a tablespoon of shredded mozzarella and a teaspoon of parmesan cheese.
Grill the pan of eggplant, uncovered, until cheese is fully melted, about 3-4 minutes.
Serve eggplants stacked on top of one another and sprinkle with chili flakes to serve if you wish.