1 cup medium tapioca pearls (look for sabudana at an Indian grocer)
3 or 4 small-medium yellow potatoes (about 8 ounces), such as Yukon Gold or Yellow Finn
Fine sea salt
1/2 cup raw peanuts
4 to 5 Thai chiles, stems removed, roughly chopped
1 (3-inch) piece fresh ginger, peeled and sliced into 1/4-inch coins
3 tablespoons neutral oil, such as canola
1 teaspoon cumin seeds
1 to 2 teaspoons granulated sugar
1 to 2 tablespoons freshly squeezed lemon juice
2 tablespoons roughly chopped cilantro, for garnish
Place tapioca in a large bowl and cover with water. Swirl to release starch, then drain. Repeat a few times until water runs mostly clear, then drain well. Return tapioca to bowl, along with 3/4 cup water. Soak for 4 to 5 hours, or until water is mostly absorbed and tapioca pearls are easily squashed when pressed between your thumb and forefinger. Drain well, and place into a large microwave-safe bowl.
Place whole potatoes in a small pot and cover with water. Season generously with salt and set over high heat. Bring to a boil, then reduce to a simmer and cook until completely tender, about 25 to 35 minutes, depending on size. Remove potatoes from water, allow to cool, then peel, and dice into 1/2-inch cubes. Add to tapioca.
Set a frying pan over medium heat and add peanuts. Swirling constantly, toast them until shiny, aromatic and golden brown, about 4 minutes. Pour immediately onto a plate to prevent overcooking. Wipe pan, and return to stove.
In a food processor, pulse chiles and ginger together until finely minced, stopping to scrape down the sides with a rubber spatula once or twice. (If you don’t have a food processor, you can do this step by hand.) Remove 1 tablespoon chile-ginger mixture, and set aside. Add cooled peanuts to food processor, and continue to pulse just long enough to roughly chop nuts. Scrape peanut-chile mixture into tapioca-potato mixture.
Heat frying pan over a medium flame and add oil. When oil shimmers, add cumin. When cumin begins to sizzle, stir in reserved chile-ginger mixture, and allow to sizzle but not brown, then immediately pour into tapioca mixture. Add 1 teaspoon sugar, toss and season with salt to taste.
Microwave tapioca mixture on high for 2 minutes, then stir. Continue cooking for another 2 minutes, then stir again. Cook for another 1 to 2 minutes in 15-second increments, until tapioca is translucent and chewy, but not quite transparent and clumpy.
Taste and adjust seasoning with salt, sugar and lemon juice.
Garnish with cilantro, and serve hot.