Drain the soaked moong dhal, place in a cooking pot or heavy-bottomed pan, add the turmeric, and pour on the 4 cups water; do not add salt. Bring to a boil and cook for about an hour until tender, adding more water as necessary (see above headnote).

In a separate pan, heat the oil until hot. Add the cumin and mustard seeds and briefly fry until popping. Tip in the onion and garlic and fry until a rich golden color. Add to the dhal and stir in.

Season with salt and pepper to taste and then add the tomatoes, carrot, and coconut milk. Simmer all together very gently for about 20 minutes, until the carrot is thoroughly cooked through. Serve in warmed bowls, with some chopped cilantro sprinkled on top, if you like.