Here is the fragment again:
Assaulted with green peas catapulted across the table from her brother's
spoon.
You correctly identified the fragment as a participle phrase. Assaulted, the past participle that begins the phrase, is the clue.
To fix this fragment, you could add a subject and verb, like this:
Mary Lou Ellen was assaulted
with green peas catapulted across the table from her brother's spoon.
Or you could use Punctuation Rule 3:
Main Clause + , + Participle Phrase.
The correction would look like this:
Mary Lou Ellen ducked under the table,
assaulted with green peas catapulted across the table
from her brother's spoon.
Or you could use Punctuation Rule 4:
Participle Phrase + , + Main Clause.
And the correction would look like this:
Assaulted with green peas catapulted across the table
from her brother's spoon,
Mary Lou Ellen fired back with a scoop of mashed
potatoes.
Enjoy your crown!