Thinly slice the scallions on the bias, separating out the dark, tender greens from the whites and light greens. Measure out 1 packed cup of the tender greens and set aside for the pancakes. Add the remaining tender greens, along with the whites and light greens, to a small saucepan. Add the oil and place over medium-low heat. Cook, stirring occasionally, until the oil begins to rapidly bubble, 5 to 7 minutes. Continue to fry, stirring more frequently now, until the bubbling begins to subside and the scallions turn crisp and golden brown, 15 to 20 minutes. They’ll burn quickly at this point, so watch carefully.
Strain scallions through a fine-mesh sieve set over a pint-sized heat-proof glass measuring cup or jar, then cool to room temperature, reserving the fried scallions to use as a garnish on fried rice, omelets, etc. When cool, measure out 1/2 cup of the scallion oil, then save the remainder for another use. (I like to stir fry with it, but swirling it into mashed potatoes isn’t a bad idea either.) The scallion oil will keep for about 1 month refrigerated in an airtight container.
For the dough, mix the flour and salt together in a large bowl. Pour in the boiling water all at once and stir with a spatula or chopsticks until just cool enough to handle, then knead by hand, in the bowl, until the dough comes together. Transfer to a clean counter and keep kneading until the dough is soft and smooth, 3 to 5 minutes. Lube the bowl with nonstick spray, add the dough, toss to lightly coat, cover with plastic wrap, and let rest for 1 hour at room temperature.
Divide the dough into four 100- to 110-gram pieces (about 3 3/4-ounce) and roll into smooth balls and cover with a kitchen towel. Working one at a time, roll each ball into an 8- to 9-inch circle measuring less than 1/8-inch thick. Brush with 1 teaspoon of the cooled scallion oil and then sprinkle with 1/4 cup (about 15 grams) of the scallion greens. Roll into a tight snake, being sure to avoid creating any air bubbles inside. Wrap the snake into a tight pinwheel with the seam facing inward. Tuck the end under and place on a parchment-lined baking sheet. Repeat with the remaining balls. Cover with plastic wrap and let rest again for 30 minutes at room temperature or overnight in the refrigerator. (If chilling, bring to room temperature before proceeding with the next step.)
Brush your work surface with 1/2 teaspoon of the scallion oil. One at a time, roll the spirals into 7- to 8-inch pancakes. Return to the baking sheet and top with a piece of parchment. Repeat with the remaining dough spirals, greasing the counter with additional oil as needed and placing a piece of parchment between each pancake.
Heat a 10-inch cast iron skillet over medium-low heatIf you’re measuring, the pan should be between 380 and 400°F. for 5 minutes. Line a baking sheet with an upside-down wire rack and stash near the cooktop.
Add 2 tablespoons of the scallion oil to the hot skillet, followed by one of the pancakes. Fry, flipping every 30 to 45 seconds, until golden brown on both sides, 3 to 5 minutes total. Transfer to the prepared rack. Repeat with the remaining pancakes, adding an additional tablespoon of oil to the pan in between each batch. Slice into wedges and serve hot.