Step 1 Preheat oven to 425°. Line an 8” or 9” cake pan or pie plate with parchment paper (alternatively, use an 8” or 9” cast iron skillet); set aside.
Step 2 Using a mandolin if you have it (a sharp knife if you don’t), thinly slice potatoes (no need to peel them). Don’t measure or anything, but they should be about ⅛” thick. While you have it out, use the mandolin to thinly slice onions, too (a knife also works).
Step 3 Drizzle a little fat of your choosing onto the bottom of the pan and arrange potatoes in a concentric circle, overlapping slightly. If you were to also just scatter the potatoes in a thin, even layer, that would work too (no pattern required).
Step 4 Scatter a few slices of onion over the potatoes, drizzle with more melted fat and season everything with salt and pepper. Repeat with remaining potatoes, onions and melted fat, gently pressing down and seasoning with salt and pepper as you make each layer, ending with a layer of potatoes and drizzle of fat.
Step 5 Place the pan of potatoes on a sheet pan to catch any drips and cover the pan with aluminum foil. Bake until the potatoes are completely tender and cooked through, 50–60 minutes. Loosen the foil, and, if you can stand it, place a dish towel on top and gently press the potatoes down as much as possible. This compresses them, ensuring even contact on the bottom and even thickness throughout. They’ll be hot, but the foil and towel will protect your hands (mostly). You can skip this step, too.
Step 6 Remove the foil and continue to bake until the potatoes are browned on the top and the bottom, another 30–35 minutes. Remove from oven and let cool slightly before unmolding onto a serving plate, removing parchment, and topping with flaky salt. Slice into wedges.
