Remove the puff pastry from the freezer and allow it to thaw for about an hour before beginning.
Peel the onions and slice them into 1 cm thick pieces, and then place them in a shallow, heat-proof pan and pour boiling water over them to soften them, and let sit for about 10 minutes. Drain the water and place the onions on a paper towel to dry.
In a heavy-bottomed pot, place the sugar along with 1/4 cup of water and turn the heat up to medium-high. Don't stir it, but you can swirl the pan occasionally. Let this bubble away and cook until it becomes caramel-coloured. You'll need to watch it carefully, especially towards the end, as it goes from zero to burnt very quickly. This should take about 5 minutes, maybe a little more.
Remove the caramel from the heat and carefully add the wine, butter, vinegar, pepper and miso. It will bubble up and react, so be careful. Whisk until it's smooth and incorporated, placing it back on low heat if needed.
Preheat oven to 425.
Pour your finished caramel into the bottom of a 8" round pie pan or cake pan.
Arrange the onion slices in a pretty pattern over the caramel.
Unroll the puff pastry, and using a paring knife, cut a circle the same size as the pie pan.
Place the puff pastry over the onions and tuck in the sides. Add the scraps to the top.
Bake in preheated oven for about 20-30 minutes, until the top is golden brown.
Remove from the oven and allow to cool for about 10 minutes.
Carefully place a plate overtop of your pan and then flip it so the tarte is on the plate and the caramel is at the top.
Garnish with herbs and serve warm.