Not everyone eats caviar the same way...however I can tell you what the traditional Russian way of eating black caviar is since I am originally from there:

White bread (toasted or untoasted), a little bit of butter, and caviar on top.

LH and most other carriers that offer caviar service will offer chopped egg, sour cream, cheese which is fine.

However, people that mix onion (especially red onion) with caviar simply means they don't like caviar. The onion kills the flavor IMO.