To make the mustard-chilli paste, soak the dried chillies in warm water for 30 minutes to soften.
Grind the mustard seeds using a spice grinder or mortar and pestle.
Transfer to a blender, add the drained softened chillies together with ½ cup (125 ml) water and blend to a smooth thick paste.
Soak the eggplant chunks in water for 10 minutes to remove any bitterness, then drain and pat dry with paper towel.
Heat the oil in a kadhai or wok and, when hot, fry the eggplant in batches until softened but not coloured. Drain on paper towel.
Tip all except 2 tablespoons of the oil out of the pan. Add the mustard-chilli paste and fry, stirring constantly, for 10 minutes until fragrant and starting to colour.
Stir in the curd, lemon juice and salt and cook for 5 minutes. Return the eggplant to the pan and stir to combine, then cook over a medium heat for another 10–15 minutes. Serve hot.