Years ago my Mum used to produce a side dish which we had with sandwiches, cold meats etc. It consisted of sliced Spanish onions and sliced cucumber which she marinaded in brown sugar and malt vinegar (I think.) It was the only way that I would eat cucumber and I still won't eat it now. I would like to try to replicate the dish but: (a) I don't know if there were any other ingredients or even how she prepared it - I have a feeling she heated the liquid but I am not sure and (b) Spanish onions seem to be a figment of my imagination tho' I guess any sweet mild onion would do. If anyone has any comments / ideas I would be grateful.
My mother did the same and it was definitely the large, white Spanish onion and none other that she used.
Sadly I can't help with quantities or method but I'm sure there was no faffing with heating the liquid.
I don't remember Spanish onions as being mild and sweet, rather to the contrary. I like them though. All I do is slice some onion, red or white, and soak in malted vinegar.
My mother used to make pickled onions and cucumber by slicing then thinly and covering them with malt vinegar which was the only available vinegar on the market at the time. Sunday night tea was not complete without them