Rinse the split peas in cold running water. Put them in a pan with the turmeric and water and bring to the boil. Cover with a lid, then simmer for 30 to 40 minutes, till the pulses are almost soft. (Watch the liquid level, adding more if you need.) Partially mash the peas with a wooden spoon or potato masher. It is especially satisfying when it is thick and slightly gloopy. To finish, melt the butter, then stir in the cumin seed and ginger and let the cumin seed become warm and nutty smelling before adding the lime juice, the chopped chillies and the coriander. Put the peas in a bowl and pour over the sizzling spice butter.