Blend one medium size tomato into a smooth paste.
Mix red chilli powder and turmeric in ¼ cup of water in a bowl until combined nicely. Keep it aside.
Heat oil and a tablespoon in a kadhai over medium heat. Once hot, add a cumin seeds and mustard seeds. Allow them to sizzle.
Add the chopped onions, ginger, garlic, and green chilli to the kadhai. Sauté until they turn golden brown. This process takes about 5 -7 minutes over low heat.
Add the chopped tomatoes to the kadhai along with salt. Mix and cook over low heat till tomatoes break down completely.
Once tomatoes are mushy, add tomato paste, spices dissolved in water and stir well to combine. Cover the pan and let the tomatoes cook down for about 10 minutes, stirring occasionally, until it form a thick, saucy consistency.
Once the tomatoes are cooked down, stir in water and garam masala.
Let the sabji simmer for another 5 minutes to allow the flavours to meld together. Taste and if required, add more salt or spices.
Just before serving, add a generous handful of sev to the tomato gravy, stirring gently to combine. Be careful not to add the sev too early, as it will lose its crunch.
Garnish with freshly chopped coriander leaves.
Serve Sev Tamatar Ki Sabji warm with phulka.