Wash the eggplants and set them aside. Pour water into a deep bowl and add about 3 tbsp. salt and mix it in. Cut the eggplants into slices, discard the top and bottom. Immerse the slices immediately into the salty water. This prevents them from browning and removes any bitter flavour from the eggplant. Let them soak for about 15 minutes. Drain, rinse them out under running water to remove excess salt, pat them dry and deep fry until slightly golden, about 5 minutes. Set them on kitchen towels/tissues to blot and dry.
Next, either 'boil then bake' or 'fry' the sliced potatoes. If you're using the first (healthier option), immerse the potato slices in a pan of slightly salted water, bring to the heat and boil for about 2 minutes. Then drain out the water and blot them dry, drizzle a tiny bit of oil and bake them on a greased tray at 170-180 C until they are tender but firm. If you're frying, just deep-fry them until done and drain on tissues to blot out excess oil.
Heat the 2 tbsp. of oil in a pan, add the onions and fry until almost golden. Next add the chopped tomatoes, garlic and all the powdered spices except for garam masala. Cook until the tomatoes are mushy, add the tomato paste and the warm water. Simmer on low heat for a few minutes until the gravy thickens and reduces slightly, and add the green chillies and some salt.
Next add the cooked potato slices, fried eggplant slices and sprinkle the chopped coriander, garam masala and lemon juice. Stir together with a light hand so that the masala coats everything properly. Try not to over mix otherwise the potatoes and eggplants will all go very mushy as they are already cooked. Adjust salt and spice to your taste, turn of the heat and serve!