The most popular story on the origin of the onion tart is that it was originally a baked good with which the bakers of old would test the heat of their ovens. If they pushed the embers aside and put one down (among the still-burning embers) and it baked up in a certain amount of time, then the oven was hot enough for bread. I tend not to put much stock in stories like this, since preparing an entire tart seems like a rather labor-intensive way to take an oven’s temperature. The classic method, as I understand it, was to toss a handful of flour onto the floor of the oven and see how long it took to burn. Thank God I live in the modern era, so I can simply point a laser beam at the thing.
And in case you’re wondering, yes, I plan to use the brick oven for my tart. I’ve never tried it before, so it should be, at the very least, interesting. Keep your fingers crossed.