Cut the sweet potatoes and the peeled onion into chunks. Grate them using a box grater or a food processor. You should end up with about 10 cups of potato and 1½ cups of onion.
Place the grated mixture in a colander set in the sink. Sprinkle it with ¼ tsp salt and toss well. Let it rest for 10 to 20 minutes, using the full 20 if your salt is coarse.
Work in batches of about one-third. Wrap each batch in a clean towel and squeeze hard over a small bowl. Move the squeezed mixture into a large bowl and repeat until everything is done.
Let the squeezed liquid sit for about 5 minutes. A pale layer of starch will settle at the bottom. Pour off the water and keep that starch for the next step.
Scrape the starch into the large bowl of shreds. Add the eggs, matzo meal or panko, cornstarch or potato starch, the remaining 1½ tsp salt, and the pepper. Mix well with your hands until everything is evenly coated.
Line a baking sheet with paper towels or set a wire rack on it. Pour oil into the skillet until it comes about ¼ inch up the sides. Heat over medium-high until it shimmers, then test with a tiny bit of batter, which should sizzle right away.
Drop 3 to 4 portions of ¼ cup each into the pan. Flatten each with a fork to about 4 inches wide. Fry for 3 to 5 minutes until deeply golden, flip gently, and fry 3 to 5 minutes more.
For bite-sized latkes, use 2 tablespoons of batter per portion. Flatten each to about 2 inches wide. Fry about 2 minutes, flip, and cook 2 to 3 minutes more.
Move the cooked latkes to your lined baking sheet. Sprinkle them right away with flaky salt or fine kosher salt. Add more oil to the pan before each new batch so it stays about ¼ inch deep.
Let the latkes cool for about 5 minutes before serving. To serve everyone at once, place them on a wire rack over a baking sheet. Keep them warm in a 300°F oven while you finish frying.