Bring the sugo to a lively simmer. Add browned eggplant back into the sauce and fold gently (you want cubes intact).
Add drained pasta directly into the pan. Add about 120 g / 4.2 oz reserved pasta water to start. Toss constantly over medium-high for 1–2 minutes.
The sauce should turn silky and cohesive, coating each curve of casarecce. Add more pasta water in small splashes if needed.
Taste and adjust salt (remember ricotta salata is salty).
Add the red wine vinegar, black pepper, and chili flakes if using. Cook for 1 minute, then remove and discard the garlic.