Enhancing garlic and tomatoes on toast I often make the following vegan breakfast toast. I start by finely chopped onions and garlic and heat the combination on a non-stick pan till the onions are brown. Then I add a tomato chopped into small pieces, with some of the juice that comes out. The outcome is a thick cohesive paste that tastes (at least to me) excellent on a breakfast toast sprinkled with salt, pepper, herbes de provence/oregano/powdered coriander.

I like it so much that I would love some pointers on how this recipe could be improved, or if this preparation has a name that I can look up and research. Specifically what I am looking for is a way of making it less dry. I plan on trying olive oil and avocado, but any tips would be welcome.
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 &amp; 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 flakes.  

As for the problem of it being too dry -- just cook it less.  So long as the bread's toasted, and you're eating it shortly after assembly, a little extra moisture won't hurt.  You can adjust the cut of the toast (if you're slicing it yourself), or cut it in half, so that it's less likely to flex if it starts to sog, or add some barrier (eg, olive oil) to the bread first so the liquid can't soak in.

You could also try roasting the tomatoes to dry them out and help activate their pectin to gel the remaining liquid.  (or add extra pectin)