Dissolve tartar powder in water.
Sieve gramflour in a bowl, add rest of the water to it and mix well to break all lumps. Add oil, tartar dissolved in water and vegetable soda. Mix well.
Pour the gramflour mixture in a flat tray and steam for 15 minutes in a steamer or a double boiler with a lid on top. Remove and cool.
Make a fine paste by blending together the cumin seeds, green chillies, green coriander, coconut, juice of two lemon, ginger and salt. Keep aside.
Heat the remaining oil in a pan, add mustard seeds and saute till they crackle. Add slit green chillies, two cups of water. Bring to a boil and pour this over the cooked dhokla. Keep aside to soak.
Slice the dhokla into two portion. Apply the prepared paste evenly on one half and place the other half back on top.
Cut into square dices and serve.