Preheat your oven to 400⁰F/200⁰C, combination steam setting. If you have variable steam settings, use 50-60%. If not, don't worry! Just set to combi steam at the correct temperature and the oven will figure out the humidity for you.
Use a pastry brush or a spray bottle to moisten one sheet of pastry, then put the other sheet on top and gently roll them together. You aren’t trying to make it a lot bigger, just creating a thicker piece of pastry for more structure in your finished tarte. Once you’ve done this, use a sharp knife to trim off the corners and make it a bit less square. Imperfect trimming is fine, and you don’t need to cut an exact circle as you’re going to tuck all the excess bits in later anyway. Put the pastry in the fridge until you need it.
Now for the caramel. Sprinkle the sugar evenly over the base of your pan and put it in the oven for about 5 minutes or until it’s melted and turning a dark amber colour (see notes). Remove the pan from the oven and, working quickly to stop the sugar cooking further, drop in the butter and salt. It will spit and fizzle – let that die down then give the pan a gentle swirl to mix the butter and sugar together. If it starts to harden, don’t worry as it’ll melt again once it goes back in the oven.
Arrange the apple pieces cut side up in the pan so they are packed in quite tightly. Lay the pastry over the top and quickly tuck the edges down around the fruit. It’s kind of like tucking a blanket around a sleeping child, and about as delicious. ;) Cut a small cross in the centre of the pastry to release steam, then put the pan back in the oven and cook for 25 minutes, or until the pastry is puffed and golden brown all over.
Remove the tarte from the oven and immediately put a large plate upside down over the top of the pan. Use a folded tea towel or oven mitts to make sure you’re holding everything safely, then flip it over and invert the tarte onto the plate. The quicker the better, you want all the sauce to end up on top of the tarte, not running out the side of the pan as you grapple with turning it. Lift off the pan to reveal your beautiful creation - I like to give the bottom of the pan a sharp tap with a wooden spoon to encourage all the fruit to come out, but if any bits remain just place them back on the tarte after you’ve turned it out.
Serve immediately with ice cream or thick cream.