So a few weeks ago, I made a ton of onion rings and they came out tasting a little fishy. This happened to me once when I made Weiner Schnitzel as well.
Here's what I did step by step -- maybe someone can help identify what went wrong:
Standard breading procedure on normal onions -- first I dredged them in seasoned flour, then dipped them in eggs, then breadcrumbs. These were regular Progresso breadcrumbs (that may have been a little stale). I put them in the fridge until frying time.
I let vegetable oil get up to temperature (I didn't use a thermometer), and did my frying. I don't think the oil was ever compromised -- it never got too hot or smoked, nor had it ever been used before. I cooked them until they looked great, salted them and they were weird tasting. Hmm.. Also, I was using a cast iron vessel (at least when I did the Weiner Schnitzel -- not sure about the pot for the onion rings), if that could affect it negatively.
What went wrong? Please let me know so I can get back on the horse that is frying.