In my experience, I've accidentally heated lettuce when making some kind of salad which makes it really bitter. Because of that, I've thought of lettuce as only fresh food. Is that not the case then?
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.
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.