Put the lentils in a pan with a litre of water, half the ginger, the turmeric and the chopped garlic. Bring to the boil then turn the heat down and simmer for 15 minutes, stirring occasionally. The lentils will collapse into a purée. You can make this thicker by simmering it more, or thinner by adding water. Stir in the spinach leaves; they’ll just wilt in the heat.
If you want to make the raita, put the yogurt into a bowl and add a little salt, pepper and about 75ml water to thin the mixture. Heat the oil in a frying pan, add the mustard seeds and cook until they pop, about 45 seconds. Add the ginger and fry for 15 seconds.
Stir the beetroot, carrot and spring onion into the yogurt without mixing it too much (it’s good when the beetroot just marbles the yogurt, before it dyes it pink). Spoon the mustard seeds, ginger and oil over the top.
To make the tarka for the dal, heat the oil or ghee and cook the onion until golden brown, then add the mustard seeds, cumin, chilli and sliced garlic.
Let this fry for a couple of minutes, then add the tomatoes. Turn the heat down a little and let the tomatoes become flavoured and soft (it will take a little longer for this to happen if you’re using fresh tomatoes). Season and add the garam masala.
You can add the tarka to the dal now, stirring it in, or you can divide the lentils between bowls and spoon the tarka on top. Add chopped coriander, if you like, and serve with rice or Indian bread and the raita, if you’re using it.