In a large saucepan, heat the oil over a moderate/high heat until hot. Add the mustard seeds and cumin and sizzle for 30 seconds until the seeds start to pop.
Add the onion and fry for 2-3 minutes until they start to darken around the edges. Add the garlic, ginger and chilli and stir briefly before adding about 1/4 cup water. This will avoid them sticking. Let the water reduce until everything is frying again then repeat this 1/4 cup water method twice more.
Add the turmeric, chilli and tomatoes and stir well for 3-4 minutes until the tomato turns into a paste.Stir in the potatoes, eggplant and salt along with 400ml water. Bring to a simmer then reduce the heat to low. Place a lid over the pan and cook for 30 minutes or until the potatoes and eggplant are very soft. Some of the eggplant will break up, this will help thicken the sauce. Remove the lid, and if needed, simmer the sauce for 5-10 minutes to reduce to a dryish thick paste. Remove from heat.
Squeeze over the lime juice, stir in the fresh coriander and serve with Indian breads or basmati rice, on its own or as part of a larger Indian offering.