Place dahl in a bowl and pour over cold water until it is 3cm above dahl. Leave to soak for 1-4 hours, but not more - it will make the dahl hard (I have no idea why this is, but it's true - I've tried it).
Drain the dahl. Place dahl in a pot with garlic and ginger.
Add cold water to come up to about 3cm above dahl. Boil on medium heat for 10-15 minutes. If the mixture gets too dry, add more hot water - cold water will slow the cooking process.
Add carrots, potatoes and drumstick and boil until all the vegetables are tender.
Soak tamarind in hot water for ten minutes, then strain and reserve the juice. Discard seeds and pulp.
Add curry powder, chilli powder, tumeric powder and stock powder and stir. Add half the tamarind juice and taste for balance - it should be ever so slightly sour. Add more to taste.
In a separate frypan, heat 2 tbsp oil. Add mustard seeds and dried chili - cook for 15 seconds, then add sliced onion. Fry until onion is lightly browned on the edges, then add curry leaves and turn off the heat.
Add this mixture to the dahl pot and stir through. Cook for a further 2-5 minutes and serve.