Add oil, onion, garlic, ginger and salt to the pot and cook, stirring, until softened, 3 to 4 minutes. Stir in eggplant, cauliflower, tomatoes, chickpeas, water and the reserved spices. Bring to a simmer. Cover, reduce heat and cook, stirring occasionally, until the vegetables are tender, 15 to 20 minutes. Top each serving with a dollop of yogurt, if desired.