Toss eggplant with salt in a colander and let sit 20 minutes. Pat dry. Heat 4 tbsp oil in a skillet over medium-high and fry eggplant in a single layer until deep golden, 6 to 8 minutes. Drain on paper towels.

Wipe the skillet, reduce heat to medium, and add remaining 2 tbsp oil. Cook onion until soft, about 5 minutes. Add garlic and pepper flakes, cook 30 seconds.

Pour in crushed tomatoes and a pinch of salt. Simmer over low heat until thickened, about 15 minutes.

Meanwhile, boil rigatoni in salted water until 1 minute shy of al dente. Reserve 1 cup pasta water, then drain.

Add pasta and fried eggplant to the sauce. Toss over medium heat, adding pasta water as needed to create a glossy coating, 1 to 2 minutes. Off heat, stir in most of the basil. Serve topped with ricotta salata and remaining basil.