I do it by ear, er, tastebud but strongly prefer the Italian-style
marinated seafood thing to the mayo-clad American-style.

Ingredients to include various shellfish thingys, lightly steamed,
for example small squid rings and tentacles, smallish shrimp, mussel
or small clam meats, scallop muscles (not roe - saute those
separately). I've had baby octopus at Italian restaurants but
would prefer not to deal with them in my kitchen. No finfish, which
I think go soggy in dressing. Baby mushrooms, a sprinkle of minced
(fresh) onion bits, a few thin slices of celery (restaurants tend to
use too much; I leave it behind). A vinaigrette made with good
white wine vinegar and mild olive oil (not EV), lightly herbed (I
don't use dill or basil in this context, but anything else goes,
including fennel fronds) and - here's where my recipe diverges
from many others - very slightly sweetened with ordinary sugar.
Go light on salt and pepper. Darn, this has set my tum churning
and tastebuds crying out, and it's breakfasttime.

Buon'appetito!