3-Ingredient Caramel Apple Tart

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.