Dahi is Hindi for yogurt and aloo means potatoes. When potatoes are cooked in tangy and creamy yogurt with a few simple spices they become Dahi Ke Aloo. I call it potato and yogurt soup. My mom serves it with hot and crisp, fresh-out-of-the-griddle rotis; I served it with some fresh baguettes.
Place the potatoes in a large pot with plenty of cold water and set over high heat. Cover with a lid and cook until fork-tender, about 50 minutes. Remove from heat, drain, and in a bowl, thoroughly mash one of the potatoes. In the same bowl, mash the other potatoes to bite-sized pieces. Set aside.
In another bowl, whisk together the yogurt and water, leaving no lumps. Heat the oil in a medium-sized pot over high heat. Add the cumin seeds. Once they start to pop, add the turmeric and coriander. Mix the spices together and add the potatoes immediately. Stir to combine.
Stir in the thinned yogurt and mix well to combine. Bring the soup to a nice rolling boil and then season with salt, to taste. Remove from the heat. Serve hot or lukewarm with additional olive oil and the red pepper flakes, if using.