I made a mashed potato and sardine mix for something similar to the croquettes on your recipe website. Should I store this mixture at room temperature, in the fridge, or in the freezer?

The strong flavor of sardines would overpower the potatoes if combined and left out at room temperature for very long, which isn’t recommended from a food safety standpoint. You should keep food cold or keep warm until serving. If mixing together, I would suggest freezing the mixture until ready to heat up in the oven or fryer.

You might enjoy this unique recipe that uses fresh sardines and a Japanese turning slicer to make potato strings:

Crispy Idaho® Potato Wrapped Sardines