Here is the item again:
But saw too late that the cream had expired three weeks ago.
You correctly identified the fragment as a lonely verb phrase. Saw, the verb without a subject, is the clue.
To fix this fragment, you could add a subject, like this:
But Sammy
saw too late that the cream had expired three weeks
ago.
Or you could use Punctuation Rule 8:
Main clause + Ø + lonely verb phrase.
The correction would look like this:
Having brewed the last bit of coffee in the house, Sammy fixed
a cup but saw too late that the cream
had expired three weeks ago.
Enjoy the luxury of your vacation!