1.Heat the oil in a large pan or wok; cook eggplant and potato, separately, in batches until browned. Drain on absorbent paper.
2.Add cumin and mustard seeds to pan; cook, stirring, until fragrant.
3.Add the onion, garlic and ginger; cook, stirring, until onion is soft. Add chilli and paste; cook, stirring, until fragrant.
4.Return the potato to pan with undrained tomatoes and water; simmer, uncovered, until potato is tender. Return the eggplant to pan with spinach and sugar; simmer, uncovered, until spinach is wilted.