Separate the onions into rings and put in a large bowl. Toss with 1/2 cup of the flour and let stand for 5 minutes.
Put 1 1/2 inches of vegetable oil into a large, wide pot over medium-high heat. Heat the oil to 365 degrees F on a deep-fry thermometer.
In a large bowl, whisk the remaining 3 cups flour together with the cornstarch, cayenne, garlic powder, onion powder, paprika and a big pinch of kosher salt. Add the milk, seltzer and egg and whisk to combine.
Add a couple handfuls of the onion rings to the batter and immediately place in the hot oil. Fry, turning once, until crisp and golden, 2 to 3 minutes. Return the oil to 360 degrees and repeat with as many batches as necessary until all the onions are fried.
Serve with ketchup for dipping.
Recipe courtesy of Rev Run's Sunday Suppers