Peel the onions but leave them whole. Slice each onion crosswise into rings that are about ¼ inch thick.
In a 5-quart pot or Dutch oven, heat 1½-inches of oil over medium heat until a candy thermometer registers 375°F. Line a sheet tray with paper towels or parchment paper, then place a wire rack inside. (The paper towels allow for easier clean up.)
In a large mixing bowl, whisk together the flour, cornstarch, salt, baking powder, garlic powder, and paprika.
In a large liquid measuring cup, whisk the milk, ½ cup cold water (120mL), and egg until fully combined.
When the oil is nearly at 375°F, whisk the cold liquid mixture into the flour mixture until no lumps remain.
Once the oil is hot, place a few onion slices into the batter and turn until well-coated. With a fork, remove each ring, letting the excess drip off, then carefully place it in the hot oil.
Continue working in batches to dip and fry the onion slices for 2 to 3 minutes, flipping occasionally until golden brown and crispy. Adjust the heat as needed to maintain the oil’s temperature between 350 and 375 degrees. Remove the onion rings from the oil using a spider strainer or large slotted spoon and place them on the wire rack to drain.
Sprinkle with more salt to taste and serve immediately.
Notes
For added flavor, use a seasoned salt in place of regular salt!
If the batter is too thin and does not coat the onions well, whisk in 1 more tablespoon of cornstarch or as much as you need to get the batter to coat the onion slices.