I'd be very interested to fine out more about this 'neutralization', do you mave any references for this SS?
I'd also assumed that maybe the salt just masked the bitterness, I don't disbelieve you but I'd like to find out more!
Assuming that any bittereness might be caused by the same irritant chemical (or perhaps its precurser) that causes tears when fresh onions are chopped up (and it may not be). I'd have thought that this chemical was either distroyed by the heat of the cooking process, or volatilizes during cooking. It is presumably a fairly volatile substance anyway, since it it reaches the eyes so fast!
Much of the stuff I've read on this thread had suggested to me that bitterenss may have resulted from insufficient cooking time, or perhaps that simmering too gentley with the lid on had not allowed these compounds to evaporate off.
One other 'variable' I've tried is how finely I slice the onions before cooking. I immagine damage to the onion either releases the irritant or activates an enzyme that causes its formation from a precurser. It could work either of two ways: course chopping could result in more irritant or precurser remaining in the onion, before cooking. Fine chopping could be said to increse the release of (or generation of) more of the irritant and thus 'use-up' some of its precurser before cooking. It all rather depends on what causes the bitterness. Is it the irritant sulphur-containing organic compound released by chopping? Is it the precurser to this compound (if there is one) contained in the 'unchopped' onion? Or maybe it's something else entirely!
What I can say is, in my opinion, it makes absolutely no difference to the result how finely chopped the onion is before cooking!
What I might try next time, is to boil by base in a wider pan, which would expose a greater surface area of boiling liquid to the air, perhaps this could get rid of these volatile irritants more effectively by providing a greater area for their evaporation. Assumimg, of course, that bitterenss has anything to do with this in the first place!
Anyway, enough rambling; I'm very interested to find out more about this salt business!