Grease a 7-inch cake pan with vegetable oil. In a bowl, whisk the yogurt and the gram flour together to a smooth batter.
In a food processor, blend the green chiles, ginger, lemon juice, 1 tablespoon vegetable oil, and 2 tablespoons of the water to a smooth paste. Add the chile paste, turmeric, salt, and 1 teaspoon of the sugar to the batter, and stir until well combined. Loosen the batter if necessary with a teaspoon of water to return it to the consistency of heavy cream. Add the fruit salts, and stir well to combine, then set aside for 4 to 5 minutes.
Fill the saucepan of an 8-inch double boiler with water and bring to a boil. (Alternatively, place a biscuit cutter of the same size or an upturned heatproof bowl into a large, heavy-based pan and pour enough water into the pan to reach halfway up the sides of the bowl. Cover the pan with a lid and bring the water to a boil.)
Pour the batter into the greased cake pan and carefully lower it into the double boiler. Cover the pan with a lid and steam the cake over a medium heat for 18 to 20 minutes, or until a toothpick inserted into the batter comes out clean.
Heat the remaining 1 tablespoon oil in a small pan over medium heat. When the oil is hot, add the mustard seeds and fry for 20 to 30 seconds, or until the mustard seeds start to pop. Be careful of the popping seeds and splatter of oil.
When the mustard seeds have started to pop, add the sesame seeds and curry leaves and fry for another minute, or until the sesame seeds are golden brown. Discard the curry leaves.
Add the remaining 4 tablespoons of water and the remaining teaspoon of sugar and boil for 2 to 3 minutes, or until the sugar has dissolved and the mixture resembles syrup.
To serve, turn out the lentil cake onto a serving plate. Pour the syrup evenly over the cake. Sprinkle over the fresh coriander and coconut.
*Can be found at specialty Asian markets. As with any baked cake, insert a toothpick into the middle to test if the cake is ready. If it comes out clean, it is done. Then pour the tarka (spiced oil) over the lentil cake before it cools as it will soak into the sponge more efficiently.