Salt and drain the eggplant. Lay the eggplant slices in a single layer on a baking sheet lined with paper towels. Sprinkle both sides generously with kosher salt. Let them sit at room temperature for 30 minutes. You'll see water beading on the surface—that's the bitterness and excess moisture coming out. After 30 minutes, blot the slices dry with paper towels. Do not rinse.

Preheat the oven and prep the baking sheets. Preheat the oven to 425°F. Line two large baking sheets with parchment paper.

Set up the breading station. In a shallow bowl, whisk together the eggs and milk. In a second shallow bowl, stir together the breadcrumbs, Parmesan, garlic powder, oregano, and black pepper.

Bread the eggplant. Working with one slice at a time, dip it into the egg mixture, letting the excess drip off. Then press it firmly into the breadcrumb mixture, coating both sides. Place the breaded slices in a single layer on the prepared baking sheets. Don't overlap—use two sheets if needed. These slices are thin, so handle them gently with a fork or a fish spatula.

Spray and bake. Generously spray the tops of the breaded slices with olive oil spray. Bake for 8 minutes. Carefully flip each slice, spray the other side, and bake for another 8 minutes, until the breadcrumbs are golden brown and the eggplant is tender. Start checking at 7 minutes—¼-inch slices brown faster than thicker ones.

Reduce the oven temperature. Lower the oven to 375°F.

Make the ricotta layer. In a medium bowl, stir together the ricotta, egg, Parmesan, salt, and pepper until smooth. Fold in the chopped spinach.

Assemble the casserole. Spread 1½ cups of marinara sauce across the bottom of a 9x13 baking dish. Arrange a single layer of baked eggplant slices over the sauce, overlapping slightly. Spread half the ricotta mixture over the eggplant. Sprinkle with 1 cup of mozzarella. Spoon 1½ cups of marinara over the cheese. Add a second layer of eggplant. Top with the remaining ricotta, 1 cup of mozzarella, and the remaining 2 cups of marinara. Finish with the remaining 1 cup of mozzarella and ½ cup of Parmesan. Sprinkle with dried basil.

Bake. Cover the dish tightly with foil and bake for 25 minutes. You want a tight seal so the steam doesn't escape—that trapped steam is what cooks the casserole through without drying out the top. Uncover and bake for another 15–20 minutes, until the sauce is bubbling around the edges and the cheese is golden brown.

Broil for the finish. If your baking dish is broiler-safe, turn the oven to broil and broil for 1–2 minutes, until the cheese is browned in spots. Watch it the entire time—it goes from golden to burnt fast.

Rest and serve. Let the casserole rest for 10 minutes before slicing. This lets the layers set so the slices hold together. Garnish with fresh basil or parsley.