My brother lives in Guam. He likes to try new stuff and he brings new recipes when he comes for visits. It usually revolve around interesting marinades. Being veggie i like to try a vegetarian version of what he has made. So this particular marinade as i recall he called it a finadenne marinade
Intrigued, I looked up a recipe for this. Ignoring the obvious Genius Kitchen which is in any case inaccessible in the UK, I hunted for a more authentic Guam version. I found this: https://www.guampedia.com/fina’denne’-dipping-sauce/ The website is most informative about the origins and uses of this sauce but I have to say the recipe given is a disappointment:
Fina’denne’: Dipping Sauce
2/3 cup soy sauce
1/2 cup vinegar
1/4 white onions, diced (green onions or red onions are optional)
4 peppers (donne’), dinache optional
...so soy sauce, onions and vinegar? Can that be it? You mentioned wasabi @oddduck - now that would ramp it up!