Bring a large pot of water to a boil. Wash and dry produce.

Trim ends off eggplant; slice crosswise into ½-inch-thick rounds. Lay flat and cut into ½-inch strips. (TIP: For faster prep, stack three rounds at a time before cutting into strips.) Thinly slice scallions, separating whites from greens. Peel and mince or grate garlic.

Heat a large drizzle of olive oil in a large pan over high heat. Add eggplant and a big pinch of salt (we used ¾ tsp; 1½ tsp for 4 servings). Cook, undisturbed, until one side is deeply browned, 2-4 minutes, then flip and continue cooking, stirring once or twice, until browned and softened all over, 2-4 minutes more. (You may need to cook in batches, adding more oil between batches; reduce heat if pan begins smoking.) 

Turn off heat; transfer eggplant to a plate. Wipe out pan. 

Once water is boiling, add spaghetti to pot. Cook, stirring occasionally, until al dente, 9-11 minutes.

Reserve ½ cup pasta cooking water (1 cup for 4 servings), then drain.

Heat another drizzle of olive oil in pan used for eggplant over medium heat. Add scallion whites, garlic, and a pinch of chili flakes (add more if you like things spicy!); cook, stirring, until fragrant, 30 seconds. 

Add crushed tomatoes, ⅛ tsp sugar (¼ tsp for 4 servings), and salt (we used 2 tsp; 4 tsp for 4); cook, stirring occasionally, until sauce has thickened, 5-7 minutes more.

Transfer eggplant, drained spaghetti, and ¼ cup reserved pasta cooking water (⅓ cup for 4 servings) to pan with sauce; toss until evenly coated.

Turn off heat; stir in half the Parmesan and 1 TBSP butter (2 TBSP for 4). Taste and season with salt and pepper. TIP: If needed, stir in more reserved pasta cooking water a splash at a time until everything is coated in sauce.

Divide pasta between plates; sprinkle with scallion greens and remaining Parmesan. Serve.