Preheat oven to 425° F and set rack to the lowest position.
Parboil potatoes in salted water until slightly tender, about 10–15 minutes. Drain and let rest until cool enough to handle, about 20 minutes.
Mix onions, kosher salt, and vinegar, and let onions soften as they sit. Once cool, coarsely grate potatoes using the large holes of a box grater or grater plate of a food processor.
Combine potatoes and onions and season with black pepper and salt. Oil a large cast-iron frying pan or an 8-inch cake pan and heat in the oven for 5 minutes. (If using cake pan, make sure it’s thoroughly oiled and line with parchment paper.)
Remove hot pan and add potato/onion mixture, pressing down with a spatula or large fork. The mixture should sizzle when it touches the pan. If it doesn’t, return the pan to the oven to heat up longer.
Place on bottom rack and bake until sides and bottom are crispy and brown, 45 to 60 minutes. Remove from oven and let rest 5 minutes. Run a knife around the edge of the pan to loosen rosti and then invert onto a cutting board, plate, or serving platter.
Sprinkle with coarse sea salt, scallions, and parsley to serve.