Roughly chop 2.5 cups of potatoes. Bring to a boil and cook until they are soft enough to mash. (About 15 - 20 minutes depending on how large the pieces are).
While the potatoes are cooking, begin to cook the spices. Heat up a separate pan first, then add the oil. Once the oil is warm, add the mustard seeds and dal. Continue to cook on medium flame stirring occasionally. Cook until the mustard seeds split and the dal turns golden brown, (approx. 3-4 minutes), then add everything else: the chopped onion, chili, cilantro, grated ginger, and salt.
Cook until the onion becomes translucent, (approx. 5-7 minutes). Add 3 or 4 Tablespoons of water throughout cooking, if needed, to keep the onions from sticking to the pan. Once the onions are translucent, turn off the heat and leave the pot on the stove to continue cooking.
The potatoes should be cooked completely now. Drain the water, allow to cool, and roughly mash with hands. Add mashed potato mixture to the cooked onion and spices. Mix well. You may need to add a few Tablespoons of water to create a creamy mixture that is completely cooked and combines. (It shouldn’t be dry and crumbly).
Reserve for use in dosas – or just eat as a yummy side dish!