Wash the mustard and spinach leaves and remove the thick stalks from the mustard leaves. Finely chop all the leaves. Heat the water in a thick bottomed pan and bring it to boil. Add the chopped mustard and spinach leaves and cook uncovered for approximately 15 minutes, until the mustard leaves are tender. Drain the cooked leaves in a strainer and keep a little of the filtered liquid to use later. Mash the leaves with a handheld blender, taking care to not make a fine puree. There should still be plenty of 'body' to the leaves.
While the greens are cooking, peel and finely chop the onions. Chop two tomatoes and slice the third. Heat 1 tablespoon of oil in a frying pan (skillet) and add the chopped onions and ½ teaspoon of salt. Cook gently until the onions are translucent then remove about ¼ of them and set them aside. Stir in the ginger and garlic paste to the remainder of the onions and continue to cook until they are golden brown.
Once the onions are golden brown add the chopped tomatoes (not the sliced one) and cook until the oil separates from the mixture. Add the mustard & spinach leaves, salt and red chilli powder to taste and mix well. Cook for a further 4-5 minutes. If necessary, you can add some of the reserve liquid that was filtered from the leaves for little moisture. Transfer to a serving dish and keep warm.
For seasoning, in the same pan, heat 1 tablespoon of oil and fry the reserved onions (the ones you removed from the pan earlier) for a few minutes until light brown. Add the sliced tomato, sprinkle with a little salt and lightly cook for a few minutes. Spoon this mixture over the Mustard and Spinach Greens and serve hot.