Heat oil in kadahi. Add mustard seeds when the oil is hot. Allow the seeds to splutter. Add chopped onion, fry onion till it turns light brown in color.
Now add dry spices turmeric powder and coriander powder. Cook for a while. Add ginger garlic paste now. Cook till the raw smell of ginger garlic paste is gone. Add chopped green chili and chopped tomato in it. Add salt as well now.
Cook tomato till they are mushy. Add 1/2 cup water in kadahi and cover the pan to cook the tomato in steam for 2 - 3 minutes.
Now add garam masala. Now turn the flames off. Add besan sev in it and cover the pan. Sev Tameta nu shaak is ready. Serve it hot.