To cook the tart, preheat the oven-proof pan over medium heat. To make a dry caramel, sprinkle a bit of the sugar onto the surface and wait until it starts to melt. Continue to sprinkle the remaining sugar around the edge where the sugar has started to melt. Once all of the sugar has been added, and it has started to melt and caramelize around the edges, you can slowly stir it.
Once the sugar has fully melted, turn the heat to low and let the caramel take on a rich, golden color. This will happen rather quickly. Remove the pan from the heat and add the apples, being careful not to touch the caramel, as it is extremely hot.
As you add the apples, the caramel will start to seize because of the temperature drop, but this is normal. Try to place the apples as nicely as possible, as the tart will later be flipped over for pretty presentation.
The apples will shrink considerably during cooking, so it is important to pack in as many as you can. Place the apples back over low heat and cook for a few minutes. Once the apples start to absorb the caramel and release their water, gently turn them over with a pair of tongs, being careful not to damage them.
Continue to cook for about 5 minutes or until the apples are just cooked through. Turn off the heat and push the apples together to fill in the gaps. Place knobs butter over top, followed by a bit of lemon zest.
Remove the dough from the refrigerator and place it over top, cinnamon-side down. Vent the dough in the center to allow steam to escape. Quickly place this into the oven, so the dough doesn’t warm up too much. Bake for about 12 to 15 minutes. Once the puff pastry is nice and golden, it's ready.
While it's still hot, place a large plate over top the pan. Very carefully flip the pan over to release the tart. Note: Make sure to use an oven mitt when flipping the tart over, as the pan will be very hot from being in the oven. If any pieces of apple happen to be stuck to the bottom of the pan, don’t worry. Gently remove them and place them onto the tart.
Let the tart cool slightly before serving. This delicious tart goes extremely well with vanilla ice cream.