Place the coriander and cummin seeds in your smallest frying pan and dry fry on a low heat for two minutes to release the aromas. Finely grind in a mortar and pestle and set aside. Alternatively, you can use 1tsp each of pre ground spices.
In a heavy based pan, heat the oil and gently saute the onion for 10 minutes without browning.
Add the ginger , garlic and chilli and saute for a further 2 or 3 minutes.
To the onion and garlic, add the ground coriander and cumin, turmeric, mustard seeds, curry leaves and garam masala and cook for one minute.
Add the lentils, tomatoes, stock and coconut milk, salt and pepper.
Gently simmer with the lid on for approximately 45 minutes to 1 hour ( depending on the consistancy you prefer) until thick and tender.
Just before serving, stir through the lemon juice.
Serve with basmati rice , nan bread and a little plain yogurt