Great question — answering as the UphillTech team:
When you mention that you've specifically cooked it down until the onions are brown and the tomatos have lost most of their liquid, I'd say that you might be near a jam ... and there are plenty of recipes out there for tomato & onion jam. Most call for adding a bit of extra acid to it and/or extra sugar. If you search for 'crostini con pomodori e cipolla' (Italian for "toast with tomato and onion"), most of the recipes have some other added salt component -- anchovies or hard cheeses were common ... but you specifically mentioned vegan. If it were me, I'd lean towards olives -- they'd hit both the acid and salty notes. Or maybe pickled hot peppers. Or even just a pinch or two of pepper...
Full answer by Joe: https://cooking.stackexchange.com/a/29600 (CC BY-SA 4.0).
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