Rinse the spinach and cook in only the water left clinging to the leaves after rinsing, with a little salt. Once completely collapsed, drain, leave to cool, then chop and put to one side.
Melt the butter over high heat in a saute pan but do not allow to burn. Toss in the mustard seeds and cook for a few seconds until they start to pop.
Turn the heat down and add the onions and garlic. Gently cook until completely collapsed and starting to turn golden. I have always found the smell of onions cooking in butter completely alluring and it stimulates my appetite more than anything. It symbolises the onset of supper.
Add the potatoes, tumeric and cayenne pepper. Cook for a few minutes to allow all the potatoes to absorb all the flavours, then add the spinach and season well.
Pour over enough water to cover. Simmer over gentle heat for 30-40 minutes until the potatoes are completely tender.
Taste for seasoning, remove from the heat and stir in the yogurt and coriander. Serve in deep bowls with nothing else except perhaps a paratha and some simply boiled basmati rice.