Soak the lentils overnight, then drain.
Put the lentils, salt, ginger, turmeric, salt, tomato purée and water in a heavy-based pan and bring to the boil. Remove the foam, turn down the heat and gently simmer for 45-60 minutes, or until the lentils are tender. In a pressure cooker this could take only 15-20 minutes. Add more water during cooking, if it’s getting too dry.
Peel and chop the sweet potatoes and carrot. Add these to the lentils after 20 minutes of cooking.
Thinly slice the onion and finely chop the garlic and chilli. Heat the oil in a frying pan and sauté the onion until soft. Add the garlic, chilli, cumin & mustard seeds and stir constantly for a minute, or until the seeds start popping. Add the curry leaves and garam masala, stir for a few seconds, then turn off the heat.
When the lentils are tender, using a potato masher, mash the dal (mainly the sweet potatoes) roughly.
Add the tadka and lemon juice to the dal and combine. Taste, then adjust the seasoning to your liking.
Serve with a squeeze of lemon juice, garnished with coriander leaves.