In the bottom of your chosen saucepan, spread the 5 tablespoons of sugar, 5 tablespoons of butter, pinch of salt, lemon zest, and 1/2 juice of lemon. Arrange the apples in a single layer on top of this mixture, with their core openings facing up.
Cook everything slowly over medium heat. The sugar and butter will melt and gradually transform into a syrup, eventually reducing and turning a deep golden brown. You might want to carefully turn the apples once or twice with two forks to ensure they're fully coated in the syrup. This cooking process usually takes around 25–30 minutes, but keep an eye on it until the syrup looks ready.
Once the apples and syrup are caramelized, carefully place the chilled puff pastry circle on top of the apples. Using a wooden spoon, gently tuck the dough around the apples so it lightly hugs them and touches the walls of the pan. Be mindful of the hot syrup.