Heat a 6" cast iron skillet on low. Add the butter and allow it to melt.
Add your onion. Allow the first side to cook for several minutes and begin to caramelize and soften. Flip and cook the second side for several minutes.
Add your honey, balsamic vinegar, thyme, and pinch of salt. Continue to cook both sides for several more minutes allowing the onion to soften and caramelize some more.
Remove the skillet from the heat. Place a piece of puff pastry cut to the size of the skillet over the top of the onion. Press down and gently crimp the edges.
Use a fork to poke several holes in the top of the puff pastry to create vents for the steam to release.
Bake the tart for 30-35 minutes until the pastry is puffed and golden brown.
Remove from the oven and allow to cool for 5 minutes.
Place a plate over the top of the skillet and flip it upside down, inverting the Onion Tarte Tatin onto the plate.