Shred potatoes on the large holes of a box grater. Transfer to a fine mesh sieve or colander and rinse potatoes while tossing a couple of times. Drain.
Preheat a 12-inch cast iron pan over medium-high heat.
Transfer half of the potatoes to several layers of cheesecloth and squeeze as much liquid out as possible (or alternately press with a potato ricer), spread onto a double layer of paper towels. Repeat with second half of potatoes.
Spread paper towels over potatoes and press dry. Season potatoes with salt and pepper, toss.
Add oil to pan coating bottom evenly then add potatoes and spread even. Press down on potatoes with a spatula to flatten so bottom browns evenly.
Let cook undisturbed about 3 - 4 minutes until golden brown on bottom, then flip potatoes (in several portions) to opposite side.
Let brown on bottom about 3 - 4 minutes. Toss once more and let cook until tender, about 2 minutes longer. Serve immediately with ketchup if desired.
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