Potato pancakes make for festive starters at any party. Serve them with toppers like creme fraiche, trout roe, smoked trout, and lingonberry jam.
In a small bowl, stir together potato starch, baking powder, 1 1/2 teaspoons salt, and 1/2 teaspoon pepper; set aside. Peel potatoes; place in a bowl of cold water. On the large holes of a box grater, grate potatoes and onion. Transfer to a fine sieve lined with cheesecloth and set over a large bowl.
Gather mixture in cheesecloth and squeeze moisture out into bowl. Let stand until a white paste settles to bottom of bowl, about 2 minutes. Remove sieve; pour liquid out of bowl and discard, leaving behind white paste.
Add potato-onion mixture to bowl and sprinkle with baking-powder mixture; toss to coat. Add eggs; stir to combine.
Heat 1/4 inch oil in a large, heavy skillet (preferably cast iron) over medium-high until hot but not smoking. (Test with a shred of potato; if it sizzles immediately, it's ready). Reduce heat to medium. Working in batches to avoid crowding, scoop a scant 1/4 cup of potato mixture into oil; flatten with a spatula to 1/4 inch thick. Fry, turning once, until golden brown and crisp, 3 to 4 minutes per side. Using a slotted spatula, transfer to paper towels set over a baking sheet. Sprinkle with flaky salt. Repeat, stirring potato mixture between batches, and adjusting heat as needed while frying. Serve immediately.
Alternate between onion and potatoes when grating to keep the potatoes from discoloring.