In a medium-size bowl, soak the sliced onion in the buttermilk for at least 10 and up to 30 minutes. While the onion is soaking, place the flour blend, salt, pepper and sugar in a large zip-top plastic bag. Seal the bag and shake to blend.

While the onions are soaking, heat at least 2 inches of oil in a heavy-bottom saucepan until it reaches 350°F.

Once the onions have finished soaking, remove the onions from the buttermilk and allow them drain fully. Transfer the drained onions to the zip-top bag with the flour blend. Reseal the bag and shake the onions to coat them fully in the flour mixture.

Remove the onions in batches from the bag of flour, shake off the excess, and place in the 350°F oil. Be sure not to crowd the onions in the frying oil or they will clump and not crispy properly. Fry until very light golden brown (about 3 minutes). Drain on paper towels. Serve immediately.