Place the onions and buttermilk in a sealable plastic bag. Give it a toss and release all of the air. Roll up the bag so the onions are fully submerged.
Heat 3-inches of oil in a large pot to 375 degrees F (190 degrees C). (Try to maintain an accurate and consistent temperature, otherwise, the onion straws get soggy. If the temperature is too high, the onion straws will burn.)
Mix the flour, salt, and paprika in a bowl.
Remove the onions from the bag with a pair of tongs (make sure they are broken up) and place them in the flour mixture. Fully coat the onions and toss to prevent clumping.
Place the coated onions in the oil for 2 minutes or until golden brown. Do not overcrowd the pot; cook in batches if necessary.
Remove the onions from the fryer to a metal bowl and toss with a pinch of truffle salt. A little goes a long way, so be careful. Once salted place them on a wire rack to cool a bit and drain any excess oil. This will make them crispier.