Peel the onion and cut it half through the root. Laying the cut side down on cutting board, slice the onions into half moons. Use your hands to break apart the layers and let them soak in a bowl with the buttermilk for at least 30 minutes.
In a pot fitted with a deep frying thermometer (or a deep fryer) heat two inches of oil to 375°F. .
While it heats, Stir together the flour, salt, pepper, cayenne, paprika, and garlic powder in a shallow bowl. Set aside. Transfer the onions to a colander and drain off the excess buttermilk, stirring a few times to make sure none is trapped.
Once the oil is hot and no longer increasing in tempeature, grab a handful of the soaked onions and transfer them to the flour. Stir them around until each one is well coated in the flour and they don't stick together. Shake off excess flour and transfer them to the oil. Cook for 3-5 minutes or until golden brown*
Transfer the fried onion strings to a plate lined with paper towels. Repeat until all the onions strings are cooked. Store in an airtight container at room temperature up to 5 days.
*Don't pull the onion strings out of the oil too early. The coating will look golden brown in the oil, but when you strain them out may not be cooked through enough for your liking. Check just one or two before removing the whole batch from the oil.
French fried onions stick together for one of the following reasons:
They are too wet from the buttermilk. This makes the coating gooey and the onions more likely to stick together. Don't skip the colander.
The dish that the flour was in was too small. By using a larger dish the onions can move around more to get completely coated with the flour.
There isn't enough flour. I know it seems wasteful to use this much flour, but you end up with the choice of too much flour or gooey onions that stick together.
The oil isn't deep enough. If there isn't enough oil for the onion strings to float in they will get too close to the bottom of the pan or stick out of the oil and not get crispy.
You overcrowded the pan. There should be enough room for the onions to move about freely while frying. If you put too much into the pan the onions will be forced to cuddle and then the flour coating melds together causing clumping.