Soak the dal in water for minimum 30 minutes.
Using a pestle & mortar, grind the cumin seeds, peppercorn and coriander seeds.
Heat the oil in a big pan and fry the ground seeds and the mustard seeds for a few seconds. Add the onion and garlic, turn down the heat and gently soften them for 5 minutes. Add the ginger, chopped tomatoes, curry leaves and turmeric, then stir for a few seconds.
Drain the lentils, add to the pan, together with the water.
Bring to the boil, remove the foam with a skimmer. Turn down the heat and gently simmer for 25 minutes, or until the dal is almost creamy.
Serve with a squeeze of lemon juice, garnished with coriander leaves.