Grate the onion and then the potatoes on the large holes of a box grater or in a food processor fitted with the shredding blade.

Put the onion and potatoes in the center of a clean dish towel, wrap them in the towel, and twist the towel over a bowl to squeeze out the excess liquid. Transfer the onion and potatoes to a large bowl and set aside. 

After about 5 minutes, pour off the water from the onion-potato liquid, leaving the potato starch that has settled on the bottom of the bowl behind, then scrape the starch into the bowl with the onion and potatoes. Add the flour, egg, salt, and pepper and mix until well combined.

Line a tray with paper towels and set near the stove. Heat 1 inch of oil in a large skillet over medium-high heat. To check whether the oil is hot enough to begin frying, drop a few shreds of potato into the oil: If they sizzle briskly, the oil is ready; if not, keep heating it. (If you try to dry the latkes in oil that's not hot enough, they will be heavy and greasy.)

Once the oil is hot, scoop out a tablespoon of the latke mixture and carefully slide it into the oil. Use the spoon to pat down the potatoes into a flat disk. Add about 5 more latkes to the oil and fry, turning once, until golden brown on both sides, 3 to 5 minutes per side. Transfer the latkes to the paper towel-lined tray and continue frying the remaining latkes in batches.

Serve the latkes hot, with a dollop of stracchino on each one.