Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Roll out the puff pastry sheet if needed to a 9-inch square, then cut into 9 3-inch squares and transfer to the prepared baking sheet. Use a paring knife to lightly score a ½-inch border around the edges of each square, making sure not to cut all the way through. Use a pastry brush to brush the egg yolk and water mixture over the borders of each square.
Place about 2 teaspoons of Private Selection® Aged Cheddar in the center of each square, then fan out 3–4 slices of apple and place on top. Sprinkle a bit of sage over the apples, followed by another pinch of Private Selection® Aged Cheddar and a small pinch of flaky salt.
Bake the squares for 15–17 minutes, until the pastry is puffed and golden brown. Let cool slightly before serving.