I had this problem at the beginning too. You can try the sweeter spanish onions, or, what I do is to spoon off the surface froth/layer duting the boiling stage.
I think many of the bitter chemicals are in that area. It helps anyway. Try not to stir that layer back in.
I think many of the bitter chemicals are in that area. It helps anyway. Try not to stir that layer back in.


). But I've made a spreadsheet to come up with an average of about 10 versions I found on the InterWeb that I'll start with when I try.