Peel and roughly chop the onions, garlic and ginger. Set aside.
In a pot, heat the oil and add 1tbsp of the cumin seeds, the coriander seeds, the onions, garlic and ginger. Cook on medium high heat till they become caramelised and brown - about 10-12 minutes.
Meanwhile, peel and roughly chop the potatoes. Once the onions are done, add the potatoes, all the spices (Except 1tbsp cumin seeds) and 3/4 cup water.
Turn the heat to low and cover. Allow the mixture to cook till the potatoes are soft and easy to mash - about 40 minutes
Once the potatoes are done, turn off the heat. Using a potato masher or a spoon, begin to mash the potatoes in the pot.
Add the remaining cumin seeds and chopped fresh coriander and mix through. Adjust the spices if you need to.
Your filling is done! Let it cool completely before attempting to handle it.