Place water into base of Instant Pot insert. Add cassava root sections.
Check that rubber ring is fitted inside Instant Pot lid. Place lid on Instant Pot, making sure the steam release valve is sealed. Press the “Manual” setting, and decrease the time until you reach 35 minutes.
When the Instant Pot is done and beeps, press “Cancel.” Allow Instant Pot to release pressure naturally for 30 minutes. Place a dish towel over steam valve (or use oven mitt), and open it to release any remaining pressure. Remove lid and insert, so the pot's contents begin to cool.
Cool cassava almost completely; then transfer to a cutting surface. The roots will have split open during cooking, or they will easily splay open. Remove tough stringy fiber that runs down the center of each one (see photo). Slice roots into long strips of desired thickness, about 8 fries per root.
Place fat in heavy bottom pot over high heat: ideally a cast iron skillet, but a large saucepan with heavy bottom or Dutch oven will work too (you'll just need to cook more batches if the surface area is smaller).
When fat is quite hot (sizzles when a drop of water is added) carefully add cassava strips in rows. You can crowd the pan a little, as long as none are overlapping or touching too much, and there is sizzling fat beneath and around each one. Cook cassava on first side about 5 minutes, then reduce heat to medium-high or medium (depending on how hot your fat is; no need to use a thermometer, you just don't want the fat smoking: a nice even sizzle of the cassava as it fries; see photo). Flip each strip over, using two forks or a set of tongs. If strips have a third side that isn't yet golden (some will, some won't), rotate cassava again so third side is fried. Remove cassava fries to a large plate (does not need to be lined with a paper towel, will not be greasy) one by one as they finish cooking, about 10-12 minutes per batch. They will not all finish cooking at the same time.
Repeat with remaining cassava fries. Sprinkle each batch *generously* with sea salt as you plate it.
Serve fries with extra sea salt and any optional dippings you like.
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