Preheat a deep fryer or fill a large high rimmed pan or pot with enough oil to fill it by half & heat to 170°c-340°f.
Peel & remove the tip from the brown onion, leaving the root intact. With the root facing up, slice next to the root to create 4 even slices around the onion in the shape of a cross. Slice in between each original slice to create 16 even-sized slices the whole way around. Flip the onion over & gently open it to create a blooming onion (see photo & video).
Add all the flour mixture ingredients to a bowl & mix to combine.
In a separate bowl, whisk together the egg and milk to create an egg wash.
Dust the blooming onion in the flour mixture making sure each individual layer is well coated.
Transfer the onion into the egg wash & coat well.
Place the egg-washed onion back into the flour and this time ensure everything is completely covered, getting in between each layer of the onion.
Gently place the dredged onion into the preheated oil & fry for 6 to 7 minutes or until golden brown & extremely crispy. Remove & drain over a wire rack or paper towel.
Serve the crispy golden blooming onion alongside the tangy mayonnaise. Dig in.