Combine the peeled, cored, and diced apples with sugar, lemon juice, and cinnamon in a small saucepot. Toss to coat evenly.

Taste and adjust the sugar, lemon juice, or cinnamon as desired. Bring to a simmer over medium-low heat and cook until the apples release some moisture.

In a small bowl, whisk together cornstarch and water. Add the cornstarch to the apple mixture and stir until well incorporated. Increase the heat as needed and boil for 1 minute.

Pour the mixture onto a plate and refrigerate until it is chilled. You don’t want to put hot filling in cold puff pastry, or you’ll get no puff because the butter will melt and the layers will disappear!

If using frozen, store-bought puff pastry, you’ll want to thaw the puff pastry sheets in the refrigerator before unrolling. It will only take about 30 minutes. I recommend thawing in the fridge rather than at room temperature, because they could easily become too warm and then won’t puff up.

On a lightly floured surface, roll out the cold puff pastry into a 12 x 24-inch rectangle. For optimal results, chill the dough for 20-30 minutes before cutting and assembling.

Place either your homemade puff pastry or the 2 sheets of store-bought puff on your lightly floured surface. Cut each of the store-bought pieces into quarters, roughly in half in both directions, using a pie wheel or pizza cutter.

Brush approximately half of each square with egg wash, then place a large dollop of cooled apple filling in the center. Be sure to leave about ½ inch from all the edges free from filling to ensure a good seal.

Fold each square in half along the diagonal to form a triangle, press the edges gently to seal. Crimp the edges with a fork and place on a baking sheet lined with parchment paper. Repeat with all turnovers.

Chill the shaped apple turnovers for at least 30 minutes, but ideally an hour. While the turnovers chill, preheat the oven to 400°F and line a new baking sheet with parchment paper.

Place the chilled turnovers on the room-temperature baking tray about 2 inches apart. You can alternate them so they fit more on the sheet. Brush each lightly with egg wash and then cut 2-3 slits in the top of each turnover.

Bake in the preheated oven for 20-25 minutes until golden brown and puffed. The bake time is mainly about the puff pastry baking fully because we have already cooked the filling.

Cool slightly on a wire rack and serve warm or at room temperature.