I love these “Blooming Onions” or “Chrysanthemum Onions”. For too few restaurants offer them and one London Burger chain, Black & Blue have recently stopped. I asked why & they said that food safety requirements meant the frier had to be cleaned totally each night & that, frankly, it wasn’t worth making them any more.
I’ve done them at home with moderate success. The trick, I was told, was to cut, carefully, diwn to the heel of the onion, to then put the asterisk onion into ice cubed water & they will spread as if on a hinge. Then cover then thoroughly with your batter and dip then into a fryer with kitchen tongues asap.
And you WILL get a messy kitchen & messy fryer.
Almost not worth the effort. But it’s wonderful when it works.