Wash the dal in cold water until the water runs clear (this is very important or the lentils will lose texture). Put the dhal in a pot with water to cover, and simmer covered until lentils are tender (add more water if necessary).
While the dal is cooking: In a large frying pan, caramelise the onion in vegetable oil.
While the onions are cooking, combine the curry paste, curry powder, turmeric, cumin, chilli powder, salt, sugar, garlic and ginger in a mixing bowl. Mix well. When the onions are cooked, add the curry mixture to the onions and cook over a high heat stirring constantly for 1 to 2 minutes.
Stir in the tomato paste and reduce heat, allow the curry base to simmer until the dal is ready.
When the dhal is tender, drain briefly (the lentils should have absorbed most of the water but you don't want the curry to be too wet). Mix the curry base into the dal and serve immediately.