Make sure that your fryer is big enough to stay hot when you drop a whole onion in. A little fry daddy type does not hold enough oil and you get lousy results. I would guess at least a gallon of oil and a nice thick fryer would be the minimum requirements. Buy extra onions so that if the bloomin' thing fails you can make some onion rings.
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS