Heat the canola oil in a saucepan.
Add the asafetida and cumin seeds. When the cumin sputters, add the curry leaves and onion and saute on a medium flame until the onions turn translucent.
Add the green chilies and ginger and stir for a minute.
Add the tomatoes and the sambar powder and cook, stirring occasionally, until the tomatoes turn mushy and start breaking down.
Now add the tuvar dal and salt to taste. Bring to a boil and then let it simmer on low heat for about 10 minutes until the flavors have merged. Add water if the dal gets too thick.
Garnish with cilantro, if desired, and serve piping hot!