Peel the onions and slice them into even rings (about ½ inch thick). Separate the rings gently and pat them dry with a paper towel.
In a mixing bowl, whisk together flour, cornstarch, milk, egg, salt, and black pepper until smooth. The batter should be slightly thick but still pourable.
Dip each onion ring into the batter, ensuring it is fully coated. Then transfer it to breadcrumbs or panko and coat evenly.
Heat oil in a deep pan to about 170 to 180°C (340 to 360°F). Make sure the oil is hot before frying.
Fry the onion rings in small batches. Cook for 2 to 3 minutes or until golden brown and crispy. Avoid overcrowding the pan.
Remove the onion rings and place them on a wire rack or paper towels to drain excess oil. Serve immediately for best results.
