Here is the fragment 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!