Brush eggplant slices with olive oil and season with salt and pepper on both sides.
Preheat grill to high heat.
Grill eggplant slices for 3 minutes per side until fork tender and nicely charred.
In a grill-safe baking dish, spread a layer of marinara sauce on the bottom.
Layer the grilled eggplant slices over the sauce and top with remaining marinara.
Sprinkle mozzarella and Parmesan cheeses evenly over the top.
Return to grill, cover, and cook until cheese melts and bubbles, about 3-5 minutes.
Remove from grill and garnish with fresh herbs or chili flakes if desired.
Serve hot, stacked for presentation.