Heat oil in a pan and temper with mustard seeds. After it stops spluttering, add the cumin seeds, garlic and asafoetida. Saute for a few seconds.
Now add the tomatoes and continue to fry till it is mashed. Add all the dry spices and green chilies. Give it a stir.
Add 1 - 1 & 1/2 cups water. Simmer on a medium flame till it is reduced to a desired consistency. Add the sev and mix well.