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. Leave for at least 30 minutes.
Spray a heavy pan with oil. 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, turmeric, 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.
Put eggplant pieces in a bowel and spray with oil. Put on baking pan with sides (I cover with tin foiand put in preheated oven (450 degrees Fahrenheit). Brown eggplant pieces.
Add eggplant pieces to the sauce pan, making sure to cover all pieces with the sauce. Cook gently until eggplant is cooked through (about 15-20 minutes). Stir occasionally (and very gently).
Add lemon juice and salt to taste before serving.