Preheat the oven to 350°F (180°C). Line a 9×13-inch (23×33 cm) baking pan with parchment paper, leaving some overhang so you can lift the bars out later.
Add the flour, ¼ cup (50 g) of the brown sugar, cinnamon, and salt to a large bowl.
Add the egg and cold cubed butter. Work everything together quickly with your fingertips until the mixture looks sandy with irregular crumbs. Don’t overwork it or let the butter get too warm.
Press about half of the crumb mixture evenly into the bottom of the prepared pan to make the base.
Grate the apples using the large holes of a box grater. Squeeze out some of the excess liquid with a clean kitchen towel or your hands.
Place the grated apples in a bowl with the remaining ½ cup (100 g) brown sugar and the cornstarch. Mix until evenly coated.
Spread the apple filling over the base, almost to the edges, without pressing it down too firmly.
Crumble the remaining mixture over the apples with your fingers.
Bake for 25–30 minutes, until the top looks dry and lightly golden.
Let it cool completely in the pan. Then lift it out using the parchment paper and cut it into bars or squares.
