In a Dutch oven or large pot over medium heat, heat oil until shimmering. Add onion and cook until softened and translucent, about 5 to 8 minutes, stirring occasionally.
Stir in green chili, garlic, and ginger and cook until softened, about 3 to 4 minutes longer. Stir frequently to avoid scorching. Add tomatoes, cover, and simmer until tender, about 10 minutes.
Stir in tomato paste, coriander, cumin, turmeric, sugar, and 1 1/2 teaspoons salt. Stir frequently until the liquid has evaporated, about 10 minutes. Stir in water and eggplant.
Reduce heat to medium, cover, and cook until the eggplant is tender and falling apart, about 15 to 20 minutes. Season to taste with salt (I didn't add any additional salt), garnish with cilantro, and serve with yogurt and naan bread.
Video
Notes
Green chilies: Look for long, slim "Indian" or "finger" chilis. Or, substitute serranos or even jalapeños. For a milder flavor, remove the seeds.
Ginger: Store the whole root ginger in the freezer, as-is and grate it when you need it using a box grater or microplane grater. To peel the ginger, scrape of the skin with the tip of a spoon or use a sharp knife.
Yield: This recipe makes about 6 c. of eggplant curry, enough for 4 servings, 1 1/2 c. each.
Storage: Store leftovers covered in the refrigerator for up to 4 days.