Wash the lentils in cold water until the water runs clear and drain. Put the lentils in a saucepan with water to cover; simmer for 20 minutes or until tender. Drain.
Meanwhile, heat oil in a large frying pan; fry the onion until golden. Combine the curry paste, curry powder, spices, salt, sugar, garlic and ginger together in a mixing bowl. Mix well and add to the onions then cook over a high heat stirring constantly for 1 to 2 minutes or until fragrant.
Stir in the tomatoes, reduce the heat and simmer for 10 minutes.
Add the drained lentils. Cook for 5-10 minutes or until the dhal thickens. Serve immediately.