Put the rice in a bowl with 4 cups water. Put urad dal and fenugreek seeds in a separate bowl with 1 cup of water. Cover both bowls and soak for 4-6 hours.
After the mixtures are done soaking, rinse in separate strainers. Add drained rice to a food processor with 1 cup water and blend for about 8 minutes until completely smooth. Set aside and rinse out food processor. Add drained urad dal and fenugreek with 1 cup water to food processor and blend for 8 minutes until smooth.
Combine both mixtures in large bowl. Add 1/2 teaspoon salt and mix again. Cover and let dosa batter sit on the counter for 8 hours.
Add oil to a large skillet and when hot, add the cumin and mustard seeds. They will start to pop. Cook for about 1-2 minutes. Then add in onion, garlic, ginger, and green chilis and cook for 5 minutes.
Add the turmeric, red pepper flakes, and the remaining 1/2 teaspoon of salt, and curry leaves to the pan and stir. Now add the chopped potatoes and cook for about 15-25 minutes on medium until potatoes are soft. Add in cilantro and more salt if needed.
When the dosa batter is done, add 1 teaspoon of oil to a non-stick skillet or cast-iron skillet on medium heat. Ladle about ¼ cup of the batter into the pan and spread out the batter with the ladle or a spatula so it is about 6 or 7 inches in diameter. Add a drizzle of oil to the top of the dosa pancake. Cook for about 2 minutes. The dosa will have brown outer edges and have lots of bubbles. Bottom should be brown and crisp. Remove from pan without flipping and drop in another ¼ cup of batter.
Repeat until all of the dosas are made.
Spoon about 1/3 cup of the potato filling into each dosa and wrap in a cylinder shape making one end tight and narrow and the other wide.