What is this piece of fatty thing, inside a fish (a large snapper) which was near the roe?
Those look like the ovaries.
I don't know how anyone else cooks them, but I usually prepare them with a simple Japanese recipe: simmer in a water, dashi, sake, mirin, sugar, soy sauce, ginger mix. Great companion for drinking!
A quick search and I can't find any recipes for this in English... You could search &quot;真子&quot; and &quot;レシピ&quot; to find other recipes (in Japanese). There's a few videos on Youtube as well.
(Remember to remove the fibers and membrane and remove the blood gently, then wash with cold water/run it through hot water/lightly salt to remove the smell.)