Arrange a rack in middle of oven; preheat to 400°F
Arrange puff pastry sheets next to each other on parchment so that longer sides are touching
Pinch seam together and gently roll to an 18x13" rectangle
Transfer with parchment to a rimmed baking sheet
Using a sharp knife, lightly score a 1/2" border around pastry (for a decorative border, make small shallow diagonal cuts with a paring knife)
Heat caramel in a small bowl in a microwave on high until smooth and pourable, about 30 seconds, or heat in a small pot over medium, stirring occasionally, until smooth and runny, about 5 minutes
Transfer 1 Tbsp
caramel to another small bowl; set aside
Toss apples and salt in a large bowl
Pour remaining caramel over and toss to combine
Arrange apple slices in 4 tight, overlapping rows on pastry, leaving a 1/2" border on all sides
Drizzle with any remaining caramel left in bowl
Bake tart until crust and edges of apples are golden brown, about 35 minutes
Transfer to a wire rack and let cool
Add 1 Tbsp
hot water to reserved caramel in small bowl, whisk to combine, and brush over apples with a pastry brush
Cut into pieces to serve
Tart can be made 6 hours ahead; store uncovered at room temperature and brush with glaze just before serving
