We’re gonna do the dough first. Mix together the flour, sugar, and salt in a big bowl.
Cut the stick of butter into roughly ¼’’ cubes and mix it in with a pastry cutter. You want to end up with pieces the size of small pebbles, so don’t mix it up too much.
Whisk together the egg yolk, water, and vanilla, and add it all to the butter/flour mixture with a fork. If it starts to give you a little resistance and clump up, you’re good.
Put some flour on a big cutting board or other flat thing, and dump the dough on top of it.
Roll the dough into a ball, then flatten it into a disk. Wrap it in plastic wrap and put it in the fridge for 30 minutes.
Preheat your oven to 400.
Put more flour on your cutting board and more flour on a rolling pin. Just throw the whole damn bag of flour at your kitchen Rip Taylor-style; you’re gonna want a lot of it.
Roll the dough to about ¼’’ thick, then cut it into rounds and press it into whatever fancy-ass tiny tart molds you got. And make sure to spray them with non-stick if they’re metal.
Put the little tartlets on a baking sheet and bake for about 15 minutes, until they start to get a little golden brown and toasty. Let them rest on a baking sheet for 5 minutes after they come out. You know the drill.
Now’s a good time to start on the filling.
Mix together the brown sugar and water in a big, wide pan over low heat. Let them get comfortable for a while, until they’re simmering and starting to caramelize and turn a little darker.
Slowly add in the vinegar, followed by the cider, and let them simmer and reduce for a bit, about 10 minutes.
You’re gonna do two different things here: get a smaller pan and pour a little of the gastrique you just made (yes, that’s what it’s called) in, just enough to cover the bottom.
Put the onions and a pinch of salt in, stir, and let them caramelize while you keep stirring. It should take about 15 minutes.
At the same time, put the apples in the bigger pot (remember, that’s gonna be full of most of the gastrique) and poach ‘em for about 5 minutes, until they’re limp but not mushy or falling apart. Basically, just keep an eye on them and make sure they don’t start turning gross.
Once the tartlets, mini-tarts, tartinis, whatever-the-hell-you-want-to-call-ems are cool, put some of the onions in each and layer the sliced apples on top, followed by about a tablespoon of brie per tart.
Set your oven to 350 and put the tarts in again, just until the brie melts. Shouldn’t take more than a minute or two.