Cover the dried beans generously with water and leave to soak overnight. The next day drain them and put in a pan with 1 litre fresh water. Bring to a simmer, partially cover with a lid and cook for 50-60 minutes or until tender. Stir in 1 tsp salt, or more to taste.
Pour the oil into a large pan or wok and set over a medium high heat. When the oil is hot put in the cumin and mustard seeds. As soon as the mustard seeds start to pop add the garlic and curry leaves.
Stir once, then add the spinach and dill. Stir and cook until most of the liquid from the spinach is absorbed. Add the yoghurt and continue to cook, stirring, until the yoghurt is absorbed.
Reduce the heat to low and add 350 ml water, the chilli powder and a good pinch salt. Stir then add the lime pulp to the spinach then pour all the contents of the spinach pan into the beans.
Bring to a simmer then cover and simmer gently for 10 minutes before serving.