If I eat a piece of herring on a cracker, what bracha do I make? Do we say that I'm eating it for the herring therefore it's Shehakol or because I have a cracker do I make a Mezonos?
As always, please cite your sources.
Your question already contains the answer,it depends why you are eating the cracker. Din Online has a whole article which discusses ikar and tafel,it is worthwhile to read the whole thing but I will just post what you are asking.
A selection from the article:
"c) It makes the main food edible, e.g., the main food is too strong/bitter:When one wants to eat a food that has a strong, bitter or salty taste, and one eats something to dilute it, no beracha is required on the latter, since it is only eaten to make the main food edible. For example: If one dips bread into whisky because the whisky is too strong for him, no beracha is required on the bread (unless he also wants to eat the bread). If one puts herring on a cracker because the herring is too sharp/salty for him, no beracha is required on the cracker (unless he also wants to eat the cracker)."
The mishnah berura clearly rules however that as long as you have some interest כדי לסעוד הלב - in order to satisfy your heart with the crackers/bread or any other secondary item, you would still need to make a beracha.