Peel the outer skin off the sweet onions and slice them into 1/4-inch rings. Place the onion rings in a bowl and pour the buttermilk over them. Let the onions soak for at least 30 minutes.
In a separate bowl, mix together the all-purpose flour, garlic powder, onion powder, paprika, salt, and black pepper. Dredge each soaked onion ring in the flour mixture until well coated.
Heat the vegetable oil in a deep skillet or frying pan over medium-high heat until it reaches about 350°F. Fry the coated onion rings in batches for about 2-3 minutes on each side until golden brown and crispy.
Use a slotted spoon to remove the rings from the oil and place them on a paper towel-lined plate to drain excess oil. Sprinkle with extra salt while still hot and serve warm.