Mix the flour and salt in a large bowl. Add the cold butter and rub or cut it into the flour, stopping while several flat, pea-sized pieces remain visible.

Add 120 ml of the ice-cold water and mix gently until a shaggy dough forms. Add only enough of the remaining water to bring the dough together; do not knead it smooth.

Turn the dough onto a lightly floured surface, press it together, and shape it into a rectangle. Roll it into a long rectangle, fold it in thirds like a letter, rotate it 90 degrees, and repeat for 3–4 folds.

Chill the dough for about 20–30 minutes whenever the butter begins to soften, then chill the finished dough thoroughly before rolling and cutting.

While the dough chills, cook the diced apples with the butter, sugar, cinnamon, and lemon juice for 5–8 minutes, until slightly softened. Cool the filling completely before assembling the turnovers.

Roll and cut the pastry into squares, then place cooled apple filling in the center of each square. Fold each piece into a triangle, press the edges to seal, and slash the tops to let steam escape.

Brush the turnovers with beaten egg and arrange them on a baking sheet. Bake at 200°C/400°F for approximately 18–25 minutes, until well puffed and deeply golden.