Combine whole wheat flour and salt in a bowl.
Cut cold butter into small pieces and add to the flour. Rub together with your hands until crumbly.
Gradually add cold water and knead until the dough comes together.
Wrap the dough in plastic wrap and let it rest in the refrigerator for about 30 minutes.
Peel and thinly slice the apples.
In a bowl, toss sliced apples with sugar, cinnamon powder, lemon juice, and cornstarch.
Roll out the chilled dough with a rolling pin and spread it evenly in a pie pan.
Evenly spread the apple filling over the dough.
Cut the remaining dough into strips and arrange them in a lattice pattern on top.
Bake in a preheated oven at 180°C (356°F) for 40–45 minutes.