[mrob]

Lettuce soup is traditional in some countries:

https://en.wikipedia.org/wiki/Lettuce_soup

I'm not sure if heating actually increases the bitterness of lettuce, or just the perception of bitterness because the softer texture makes better contact with your tongue. I personally am not very sensitive to bitter tastes, but if you are, you could use a smaller portion. Easy portion control is another advantage of frozen vegetables, because you don't have to worry about the unused portion rotting.