Start by cutting onions into ½ inch thick slices and separate them into rings. Don't cut them thicker, your onion rings will not be cooked throughly.
In a small sized mixing bowl add ½ cup flour and add 4-5 onion rings at the time to the bowl to coat them with some flour. Shake any flour excess and transfer them to a wire rack or on a baking paper sheet.
In a medium sized bowl, add eggs and whisk. Add 2 cups flour, cornstarch, milk, garlic powder, any dried herbs (if preferred) salt and pepper and whisk to combine until the batter is smooth.
On a large plate add breadcrumbs and set aside.
Start adding onion rings, one by one, first into the batter to give them a good coat and then onto the plate with bread crumbs. Try and coat evenly with breadcrumbs. Set aside and continue till you've finished all sliced onion rings.
In a deep frying pan, add vegetable oil and heap up on medium high heat. The oil should be very hot (around 360F) when you add the onion rings (you can test how hot it is by dropping a bit of batter and if it starts to get bubbly, the oil is perfect)
Deep fry for 4 minutes, until really crunchy, turning them half way into the time.
Remove from pan to a large plate lined with 2 layers of paper towels to remove any oil excess.
Serve with your favorite sauces: tomato ketchup, mayo, etc