Cut the eggplants into 1-inch pieces- preferably keeping at least one side of each piece with the skin on.
Put the eggplant pieces into a strainer and sprinkle liberally with salt. Place a small plate on top of the eggplant, and a weight on top of the plate- this process draws the excess water out of the eggplant, and it won't be overly salty.
Heat 1 1/2 tbsp oil in a heavy pan. When hot, add mustard seeds- they will start to pop in a few seconds. When they begin to pop, add the sliced onions, garlic and ginger. Turn the heat to medium, and cook until mixture is nicely browned.
Add cayenne pepper, tumeric, garam masala and cumin to the onion mixture. Cook spices for a few minutes, then add water and tomato paste, and reduce heat to low. Let the sauce cook gently, but don't allow it to become dry.
In a large skillet, heat 1 1/2 tbsp oil over moderate heat. Add eggplant pieces to the pan, and arrange in a single layer. Saute until all sides are brown and crisp.
Once all eggplant pieces are browned, incorporate them into the sauce pan, making sure to cover all pieces with the sauce. Add lemon juice and salt.
Cook gently until eggplant is cooked through (about 15-20 minutes). Stir occasionally (and very gently) and serve.