1. Heat oil in a deep pan to sizzling but not smoking, about 340-360 degrees Fahrenheit. While it’s heating, prepare the batter by mixing the flour, cornstarch, egg, salt, and cold seltzer. It’s OK if the batter has a few lumps; it’s important not to over-stir. Add the onions.
2. Add a few of the battered onions to the hot frying oil; put the remaining onions and batter in the refrigerator. Don’t overcrowd the pan, or you’ll wind up with greasy onions. Once they’re golden brown, remove them from the oil and let them drain on a cookie sheet covered with paper towels. Continue the process with the remaining onions, keeping the raw battered onions cold in the meantime. Salt as needed and serve hot.