Pop the potatoes into a medium saucepan and fill with cold water until covered by about 1-inch. Season well with salt. Bring to a boil, lower to a simmer, and cook until a fork inserted into the center of the potato meets no resistance, 20 to 30 minutes. Drain and pass through a ricer or food mill. Stir in the butter while hot.
Warm the oil over medium-high heat in a skillet, preferably cast iron. Add the seeds; they should sizzle upon contact. Once they stop popping, add the onions and saute until softened.
Add the mashed potatoes to the pan and flatten with a spatula so that potatoes can begin to crisp. Continue to toss and re-flatten the potatoes until there are patches of crispy deliciousness all the way through the potatoes, about 10 minutes. Remove from the heat, sprinkle with parsley, and serve.