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 tosfos on the mishnah, regarding the fruits of ginnosar berachos 44a, which were so sweet they required bread to damper the sweetness, says: ואינו בא אלא בשביל המליח, implying that the only reason you dont need a bracha for the bread is because the only reason you are eating the bread is for the sweet/salty but not because you also want bread.
In berachos 41a, rashi writes regarding radish and olives ולא אכל זית אלא להפיג חורפו implying if you wanted the olive also to dampen the sharpness of the food and because you like olives you would need to make 2 berachos.
The shulchan aruch in orach chaim ריב-א says even when not eaten directly together, if the tafel item is coming to remove the unpleasantness of the first food, like bread and salty fish make one bracha.
Says the mishnah berura: אבל אינו תאב כלל לאכול פת, לכן הוי הפת טפל. משא"כ אם הוא תאב לאכול פת גם כן, אע"פ שאוכלו עם המליח, כהנהוג לאכול דג מלוח שקורין הערינ"ג עם פת, אינה טפלה אליו, אפילו אם תאב להמליח יותר, וצריך לברך עליה המוציא ופוטר את המליח. This would seem to be very clear for our case that you would have to only be eating the bread because of the saltiness of the fish.
However the shaar tziyon, questions whether this is only for bread and that is why they bring specifically the halacha for bread, which because of its elevated status is only tafel when you absolutely dont want bread. Or is this for all ikar tafel situations.
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.