In a large cast iron dutch oven, fill with oil deep enough to cover onion. Heat to 350°F. Meanwhile, peel outer skin of onions. Slice top tip of onions off, but do not disturb the root end. With a paring knife, starting 1/2 inch from the bottom of the onion, slice down to top and continue at 1/4" intervals until onion is sliced all the way around (like a mum). Shake out each onion.
Gently pick up onion and spoon batter onto onion ensuring every part is coated, then gently allow to drain slightly. Carefully place onion root side down into hot grease and allow it to cook till light brown. Remove onion from grease with a slotted spoon and use a knife to pull apart petals that are stuck together. Place onion back into grease top side down and when golden brown, remove with a slotted spoon to paper towels to drain.
Shake on Cajun seasoning and serve with ranch dressing as a dipping sauce, if desired.
Mix together beer, flour and seasonings; let sit on the counter for several hours.