Slice the onions; boil until just tender.
Soak the clams in a small amount of vinegar while onions are boiling.
If using scallops, lightly salt them before soaking in the vinegar.
Add sugar and dashi to the miso; blend very well.
Stir in the vinegar.
Drain the clams or scallops; add, along with the onions, to the sauce.
Serve seafood immediately.