First, make the caramel. Heat a heavy bottomed sauce pan, heat the sugar on low until completely melted, continuously whisking every 15-20 seconds. The sugar will form clumps before it melts. Slow and steady to ensure it won’t burn.
Once melted, remove from heat and whisk in the butter. It will violently bubble; don’t worry, you’re doing it right. After butter is melted and mixture is thicker, whisk in heavy cream and pinch of salt. Fold until luxurious caramel is formed.
Pour into a small bowl or cup, set aside. **if caramel hardness after cooling, pop it in the microwave for a few seconds to thin again.
Then, cut each apple into fours, around the core. Slice each apple chuck into thin slices. Place apple slices into a large bowl, along with ground cinnamon, ground nutmeg, lemon juice and cornstarch. Toss together until evenly coated: tip, measure out the seasonings then sprinkle over the apples evenly for better coverage.
Preheat the oven to 425°F. Roll the sheet of puff pastry onto a parchment lined cookie sheet and poke it with a fork at least 20 times.
Layer the apples in four uniform rows on the puff pastry, leaving at least 1” border around the edges (this is crucial so everything stays in place). Fold the edges over to keep the apples in place, and use a fork to press down on the puff pastry (like a pop-tart), then use a pastry brush to brush the edges with the egg wash.
Spoon out the caramel from the bowl and place in small blobs around the pastry; saving at least half for topping after baking!
Bake for 30-35 minutes until edges are golden brown and puffed & apples are cooked through. Remove the baking sheet from the oven. Warm the remaining homemade caramel sauce and pour over top of the baked puff pastry.
Enjoy warm; with a dollop of ice cream/cool whip, toasted nuts, or sprinkle of powdered sugar.