sashimi garnish
Refers to the shredded daikon, shiso leaves, or other decorative garnishes served alongside sashimi. Literal culinary term.
I eat all the daikon garnish that comes with the sashimi too.
something insignificant; something easily dispensable
Figurative use derived from the garnish being secondary to the main dish. Often used in the phrase 刺身のつまのように扱う (to treat as trivial).
He was treated as if he didn't matter at the meeting.
That opinion is of no consequence.