If fresh mustard greens are used, remove any coarse stalks, chop finely and wash thoroughly in a few changes of water. Thaw the frozen mustard greens, if used.
Place mustard greens, red chilli, spinach and salt in a heavy based saucepan and cook over low heat for about an hour or until the leaves are very tender, stirring occasionally. ( The leaves will release water during cooking. Add more water if it dries up quickly.)
During cooking mash the leaves with a saag masher or a wooden spoon against the sides of the saucepan. (alternatively put the cooked greens in batches in a food processor or blender and blend to a puree without adding more water.
Stirring continuously cook for 10-15 minutes or until thick and creamy. Heat oil in a small frying pan add finely sliced ginger and onion and fry to a light golden color, Add green chilies, stir once and add to saag. Cook over low heat for 5-8 minutes, stirring constantly.