Heat some coconut or vegetable oil in a pan and fry the onion, garlic, ginger (and 1 or 2 chopped chillies if you want some heat) off for about 5 minutes.
Add the coriander, turmeric, cumin and cook out for a few minutes until aromatic.
Tip in the sweet potato, and the lentils.
Pour in stock and 1 can of the coconut milk. Set the other one aside.
Cook the dhal until the lentils and vegetables are soft and the whole thing is silky. It should take about 20 or so minutes.
Add the spinach at the end, once the dhal is cooked, with a squeeze of lime and plenty of seasoning to taste. At this point you may wish to make it lighter, creamier and more viscous with some more coconut milk. (I reheated mine the next day and added the second can to loosen it as dhal does thicken on cooling.)
To make the garnish, simply fry the coriander, mustard seeds and chilli flakes (leave these out if feeding kids) for a few minutes in oil. Then add the onions, salt and sugar. Fry until crispy.