Place a heavy pan or skillet on medium heat and add the oil. When the oil begins to shimmer, add the cumin and black mustard seeds. Heat until they being to pop, then add the sliced ginger and cook 1-2 minutes until the ginger is fragrant. Add the chillies, salt, ground coriander and cumin and cook for about 1 minute. Add the potatoes and mix. If the mixture is too dry add a little water to keep the potatoes from sticking to the bottom of the pan. Once the potatoes have been infused with the spices, add the spinach and cook until the spinach wilts. Sprinkle with the nutmeg and mix.