Here is the item again:
But found only a cookie bag full of crumbs and a tablespoon of milk.
You correctly identified the fragment as a lonely verb phrase. Found, the verb without a subject, is the clue.
To fix this fragment, you could add a subject, like this:
But Frankie
found only a cookie bag full of crumbs and a tablespoon
of milk.
Or you could use Punctuation Rule 8:
Main clause + Ø + lonely verb phrase.
Frankie rushed into the kitchen for a snack
but found only a cookie bag full of crumbs and
a tablespoon of milk.
Enjoy your throne!