Preheat the oven to 350°F. Place the eggplant slices on a lined baking sheet and drizzle each side with olive oil and season with salt and pepper. Bake for 10 minutes.
While the eggplant is baking, combine the mozzarella, ricotta, and Parmesan in a bowl and mix well.
Next, in a sauce pan over medium heat sauté the onion for about three minutes until caramelized. Add the garlic, chopped tomato and spinach to the pan and stir until the spinach has wilted. Add the Hunt's Fire Roasted tomatoes to the pan and stir to incorporate. The mixture should be saucy.
Arrange the baked eggplant slices in a rectangular baking dish and pour the tomato mixture on top.
Sprinkle the top of the baking dish with the cheese mixture, add pepper to taste and cook for 30-40 minutes or until the casserole is cheesy and bubbly on top.