The first step in preparing your purple onion for wokking in your jumbo shrimp and prawn wok recipes is to slice off the top and bottom of the onion and peel off any of the thin and/or decayed layers of onion skin. You should not have to worry if the one or more of the outermost onion segments are not edible, as long as the interior of the onion is in good shape you are fine. Keep an eye out for any rot on the inside in between the ring segments, but if there isn't any, then any outside blemishes or outer ring damage can be stripped off and ignored.
A Purple Onion with the top and bottom removed
You can see this is a decent onion with nice juice and some decently thick segments. This isn't as thick as we would like, but it's a pretty good one! The best onions have little of the yellow or whitish interior region which is where the onion is getting ready to sprout, and the segments are uniformly thick all around the circumference of the onion.