Pour oil for frying into a skillet and heat to 350°F (175°C).
Place 2 cups buttermilk in a small bowl.
In another bowl, combine 1 1/2 cups flour, 2 teaspoons garlic powder, 2 teaspoons onion powder, 1 teaspoon paprika, 1/2 teaspoon salt, and 1/2 teaspoon ground black pepper; whisk to blend.
Use a sharp knife to slice 2 large onions thin. Separate the slices into individual strings or strips with your fingers.
Working in batches to avoid crowding the skillet, use tongs to dredge a portion of the onion strings in the buttermilk, letting excess drip off, then transfer them to the flour mixture and toss to coat evenly. Shake off any excess flour.
Carefully add the coated onion strings to the hot oil and fry 2–3 minutes, or until golden brown and crispy, maintaining the oil temperature near 350°F between batches.
Remove fried onion strings with tongs or a slotted spoon and drain on paper towels. Repeat steps 5–7 with the remaining onions.