Onion Tartlets

Because sometimes there has to be savory mixed in with the sweet...
I've been asked for the recipe for these little onion and potato tartlets, which are very easy and quite delicious, so here it is.
You'll need:
1 package store-bought puff pastry (I'm sure there is someone who makes her/his own puff pastry at home but that is way above my pay grade).
2 medium-sized sweet onions peeled and cut into 1/4 rounds
5 or 6 small potatoes (by which I mean baby potatoes; Yukon Gold, etc.) cut into 1/4 inch rounds
Salt, pepper, a bit of rosemary or other seasoning
Balsamic glaze (recipe to follow)

To prepare:
Preheat oven to 425 degrees F.
Grease six mini tart or pie pans (4-5 inches) with butter and set aside.
I use these, with removable bottoms, but any little pans will do.

Roll out your puff pastry to about 1/4-inch thickness and cut out 6 rounds that are a little smaller in diameter than your pans. Prick the rounds with a fork and transfer them to a parchment lined baking sheet. Keep these refrigerated while you prepare the filling.
Place 2 (or more if the onions are on the smaller side) onion rounds on the bottom of each pan
Cover these with 2 layers of potatoes placed on top in concentric circles
Season the tarts with salt, pepper, and a little fresh rosemary
Place the chilled puff pastry shells on top of each tart and slightly tuck the edges under the filling
Bake until golden brown - about 25 minutes (I like my tarts on the more done side with the onions all caramelized so I left them in a bit longer).
Invert the tartlets onto a plate.
Now to make the glaze:
1/4 cup balsamic vinegar
4 tablespoons unsalted butter
1/2 teaspoon brown sugar
Combine the sugar and balsamic in a small saucepan and bring to a simmer, cooking until the mixture looks like syrup. This takes some patience - you just stir and watch for about 5-10 minutes - but worth it. Whisk in the butter one tablespoon at a time until melted and incorporated. Glaze the tarts, adding a bit more salt and pepper to season.
If you don't feel like making your own glaze, you can probably use this (I haven't used it on these tartlets, but I just made some roasted vegetables using it last night and they were fab):

Serve the tartlets warm. They won't last long.