Line 2 large baking sheets with parchment paper for easy clean up. Transfer the diced eggplant to the two prepared sheets. Drizzle ¼ cup of olive oil over top (about 2 tablespoons per baking sheet), then season well with kosher salt & ground black pepper. Toss to combine, then spread the eggplant into an even layer across both baking sheets.
Transfer the eggplant to the oven, placing one baking sheet on the top rack & one baking sheet on the lower rack. Roast 25-30 minutes, rotating the baking sheets & switching the oven racks about halfway through, until the eggplant is tender & deeply browned. Remove from the oven & set aside.
Bring a large pot of generously salted water to a boil. About 10 minutes before the eggplant is finished roasting, drop the pasta. Cook to al dente according to package directions, stirring occasionally to prevent sticking. Once cooked, carefully dip a liquid measuring cup into the pot to reserve about 1 cup of the starchy pasta water, & set aside. Carefully drain pasta.
About 10 minutes before the eggplant is finished roasting, heat the tomato sauce. To a large skillet over medium heat, add the remaining 2 tablespoons olive oil, garlic, & crushed red pepper flakes. Cook, stirring often, until the garlic is golden & fragrant, about 2-3 minutes. Carefully pour in the tomato sauce, stirring well to combine with the garlic-infused oil. Reduce heat to low to maintain a gentle simmer.
To the skillet with the warm tomato sauce, add the drained pasta & roasted eggplant, along with the thinly sliced basil & grated parmesan. Toss to combine well – the sauce should evenly coat the pasta & eggplant. Add in some of the reserved pasta water if the sauce needs to loosen up a little; add in an extra handful of parmesan if it needs to tighten up more. Cook 1-2 minutes longer, allowing the pasta to meld with the sauce, then remove from the heat.