You will need a heavy oven-proof saucepan which will roughly fit your six apples in one layer without letting them scatter around. If only five apples fit, leaving a little window in the middle, that’s not a problem: as apples cook, they will shrink some, and you should be able to tuck the sixth apple right in between.
Roll the puff pastry out to 1/4″ thickness and cut out a circle, which is about 1/2″ larger in diameter than the rim of the sauce pan. Refrigerate the dough until needed.
Preheat the oven to 400F (200C)
Spread sugar, butter, salt, zest and lemon juice (and vanilla if using) on the bottom of the aforementioned sauce pan.
Arrange apples in one layer on top of the sugary mix, with their core openings pointing up
Cook everything together slowly. Sugar mix will slowly turn into a syrup. Syrup will eventually reduce and turn its color to golden brown.
You may want to turn the apples upside down 1 or 2 times very carefully, using two forks. I decided to do so just because I wanted to be sure that syrup coats the apples all over.
The approximate time of the apple cooking for me was around 25-30 minutes, but you should let your eyes determine the final cooking time, rather than my approximation.
When you think the syrup and apples are ready, very carefully, trying to avoid the nasty burns from the syrup, pop the puff dough circle on top of the apples. Using a wooden spoon gently tuck the dough around the apples, so that it hugs them just slightly. It’s best if the dough actually touches the walls of the pan.
Place the pan in the oven and bake for about 30 minutes, until dough is nicely browned
Remove the pan from the oven and let it rest for about 5 minutes.
Place a serving plate on top of the pan, bottom up, and holding the plate tightly pressed against the pan, carefully flip the contents onto the plate.
Rearrange the apples gently if they shifted too much and let the syrup trickle down into the puff crust.
Serve warm. The tarte was very good just by itself, but if you are up for it, I suggest whipped cream, creme fraiche, or plain vanilla ice cream.