How can I reduce the bitterness of onions?
Although I've never encountered bitter onions, if something is bitter the best thing to do is counteract the bitterness with more sugar. 

If you're eating them raw I would recommend mixing sugar and lemon juice in a bowl and mixing with the onions and leave for about 20 minutes. The sugar balances out the bitterness and the lemon juice helps to take away from the "rawness" of the onions, as well as adding flavour to mask any bitterness.