Chutney: In a fry pan, dry roast peanuts to a golden brown. Grind peanuts, chilies, tamarind paste and salt to a smooth paste using 1/4 cup of water. The amount of water depends upon how thick/thin you want your chutney.
Transfer to a bowl. Heat oil until sizzling. Add mustard seeds and hing and fry until seeds start popping. Pour over the Chutney.
Filling: Boil the cubed potatoes until fork tender, drain and set aside. Heat the oil over medium high heat and add the mustard and cumin seeds. They will begin to pop and shoot all over your kitchen within a few seconds so have the chopped onion ready to go.
Once the mustard starts to pop, add the onion and green chili, sauteing for a couple minutes. Add the ground cumin and coriander and mix, gently stir in the cubed potatoes as you don't want to turn them to mashed potatoes.
Add the turmeric, garam masala, chopped cilantro and mix well. Add a little water (up to 1/4 cup) to thin the mixture and salt to taste. Keep warm.
Dosa: Place the urad dal and basmati in two separate bowls and cover with several inches of water. Cover and allow the bowls to sit out at room temperature at least overnight and up to one day.
In the morning, drain and rinse the rice and dal and add to your food processor. Grind the two together into a paste, add the onion and chili and salt (about 1/4 teaspoon) and continue to grind. Start adding water to the mixture, until you get a smooth creamy and slightly thin batter (just a tad thinner than your typical pancake batter). Continue to mix this in the processor until you can rub it between your fingers and the grit remaining should be a little finer than cream of wheat.
Heat a large flat, oiled griddle over medium high heat, or your largest flat bottom non stick pan. Pour 1/2-3/4 cup of batter into the center of the griddle and working quickly with a large spoon or heat safe silicon spatula, swirl the batter outwards into a very thin flat pancake. The thinner the better. Don't be too concerned about holes or getting it perfectly round, just focus on spreading it thin.
Cook the dosa until the underside is golden brown and the top is no longer glossy. This will only take a few minutes and do not flip the dosa. If your dosa seems a little thick, too chewy or not quite crispy enough, blend the batter again and add a little more water.
Remove the dosa from the pan and fill with several tablespoons of the following potato filling and a little squeeze of sriracha. Roll up and serve immediately with the peanut chutney.