How to reduce bitterness in silken tofu 'cream' soup?
The bitterness was likely due to one of two things: the vegetarian soup stock or the packaged fried onions.  The stock would be a like candidate if you used a concentrate or boullion, rather than stock-in-a-box, which tends to be too sweet, if anything.  Otherwise, look to the fried onions; if the ones you used are burnt or rancid, they would add an unpleasant bitterness to anything you added them to.  Try tasting the ingredients individually until you locate the bitter flavor, and then swap in a better quality ingredient.