Slice the onions very thinly using a mandoline or slice them by hand, using a knife.

Separate the onion layers and place them in a shallow baking dish or large ziploc bag. Pour the buttermilk over the onions and let them soak in the refrigerator for at least one hour. 

Whisk together the flour, dry seasonings, and baking powder in a large bowl. 

Working in very small batches, shake off just a few onions at a time and drop them into the flour, tossing until fully coated. Don’t flour too many at a time because you don’t want them to remain wet and stick together. 

Drop the onions into the oil as soon as you coat them in flour. Don't shake off too much flour because you want the coating to stay on the onions.

Let the onions fry for about 1 minute, they should be golden and crispy.

Transfer the onion strings to a cooling rack or paper towel-lined baking sheet and arrange them in a single layer.

Repeat the coating and frying process for the remaining batches of onions.