Preheat the oven to 500°F. Line a large, rimmed baking sheet with high-heat-resistant parchment paper (or nonstick foil). Spray the parchment with avocado oil spray.

Slice the eggplant into ¼-inch thick slices. Arrange the slices on the prepared baking sheet. Spray with avocado oil. Season with kosher salt, black pepper, and garlic powder. Roast the eggplant for 15 minutes.

Remove the baking sheet from the oven. Turn the eggplant slices to the other side. Spray again with oil. Return to the oven and roast until browned and tender, about 10 more minutes.

Remove the baking sheet from the oven. Reduce the oven temperature to 425°F.

Arrange half of the roasted eggplant slices in the bottom of a greased 1-quart casserole dish. Spread half the marinara sauce on the eggplant pieces and sprinkle them with half the mozzarella.

Arrange the remaining eggplant slices on top. Spread them with the remaining marinara sauce, and sprinkle with the remaining mozzarella and the Parmesan.

Bake the casserole until golden and bubbly, for about 15 minutes. Allow it to cool for 10 minutes before serving.