First of all, I must say that I live close to the caribbean, in Venezuela, thats why I usually eat a lot of fresh seafood. This is the simplest preparation of sardines, only with onions and coriander.
As usual, for me it's no about exact measures but proportions... and taste.
I use a proportion of 60% sardines (fresh and clean), 30% onions and 10% coriander. I prepare it that way to keep the sardines as the principal flavor, but with enough presence of the rest of ingredients.
The preparation, as said before, is very simple. Just clean and shred the sardines, chop the onions and coriander, and put it to cook with a little oil.