How to Make It
Bring 2 inches of salted water to a boil in a large pot fitted with a steamer basket. Add the potatoes and season with salt. Cover and steam until the potatoes are tender, 8 to 10 minutes. (Check one of the smaller ones after 8 minutes to see how tender it is; you should be able to insert a fork into it easily.) If you don’t have a steamer basket, boil them in a large pot of salted water until tender, 10 to 15 minutes.
Remove the potatoes from the steamer and let them cool slightly. Using the bottom of a bowl or cup or the palm of your hand, smash the potatoes until they’re just crushed to expose the inside, but not so much that they fall apart. You’re going for maximum crispy surface area here.
Heat the chicken fat in a large skillet over medium-high heat. Add the potatoes in a single layer (work in batches, if you need to) and season with kosher salt and black pepper. Cook until both sides are super browned and crispy, about 5 minutes per side.
Remove the potatoes with a slotted spoon or spatula and transfer them to a serving bowl or platter. Add the butter to the skillet and let it melt and foam. Add the onion rings in a single layer and season with kosher salt and black pepper. Cook, swirling the skillet occasionally, until the onions have turned golden brown and started to crispy, 4 to 6 minutes.
Remove the skillet from heat and add the red pepper flake, swirling the skillet a few times to combine. Pour the onions and any butter in the skillet over the potatoes and top with flaky sea salt and the parsley.