Position racks in upper and lower third of oven. Preheat oven to 375. Line 2 large rimmed baking sheets with parchment. Divide eggplant chunks between the prepared baking sheets and toss each sheet with 2 T of oil. Season with salt and pepper and spread into a single layer. Bake for about 45 minutes, until golden brown, rotating baking sheets halfway through baking. Keep oven on.

Meanwhile heat 1/4 of the olive oil in a large pot. Add the onion, season with salt and cook over medium low heat, stirring occasionally until softened. Add garlic, crushed red pepper and 5 of the basil leaves and cook until garlic is soft and fragrant, about 1 minute. Add tomatoes, squeezing each one with your hand and dropping into pot. Stir occasionally, smashing tomatoes as you go. Simmer about 45 minutes until sauce has thickened. There should be about 6 c of sauce.

Spread 1 c of the sauce in the bottom of a shallow 2 1/2 quart baking dish. Top with half of the eggplant and the remaining basil leaves. Drizzle with 1 T of the olive oil. Spread 2 c of the sauce on top and sprinkle with half of the cheese. Drizzle with 1 T of olive oil. Top with remaining eggplant, sauce, mozzarella and 1 T of olive oil.

Bake the gratin in the upper third of the oven for about 30 minutes or until bubbling. Remove from oven and preheat the broiler to high. Sprinkle the remaining 1/2 c of Parmesan over the top and broil for 3 minutes or until golden brown. Let cool for 10 minutes before serving. (If this has been refrigerated before baking, please bring to room temperature before placing on oven.)