Peel the Onions and carefully slice each one into discs which are between ¼" and ½" wide (see video)
Seperate the onion rings from the discs and place them in a large bowl of Ice water for 1 hour.
After the hour drain and pat the rings dry with paper towels
In a large bowl add the flour, egg, Oil and milk and whisk together until a gloopy batter forms
Season the batter with Salt and Pepper
Heat around 2" of Vegetable oil in a deep pan until searing hot (you want the onion rings to sizzle as soon as they meet the oil)
Work in batches and dip onion rings in batter until evenly coated, then place carefully in the pan, fry for around 1 minute each side until crispy and golden
Place each cooked onion ring on a paper towel to soak up any excess fat before serving
Serve while still hot