Fish in aspic sounds similar to gefilte fish, which is universally terrible in every jar, can, or whatever you can find at the store.
I wish I could tell you how, but my grandmother would make gefilte by hand annually and it was delightful. Slightly tart, slightly sweet, bouncy texture and bursting with delicious whitefish flavor and lightly spiced.
So while I don’t have a recipe, I know it can be done and maybe looking into recipes for homemade gefilte fish can help you figure out how.