Preheat your oven to 350°F (175°C). In a large bowl, combine the flour, white sugar, baking powder, and salt. Cut in the butter until the mixture resembles coarse crumbs. In a separate small bowl, beat the egg yolks with the water. Add this mixture to the flour mixture and stir until a soft dough forms. Roll the dough out and press it into the bottom of a 15 × 10-inch baking pan.
In a large bowl, combine the sliced apples with the lemon juice, white sugar, flour, and ground cinnamon. Toss everything together until the apples are evenly coated. Arrange the apple mixture over the prepared crust and dot the filling with the butter.
In a medium bowl, combine the flour, brown sugar, and ground cinnamon. Cut in the butter until the mixture becomes crumbly, forming a coarse topping. Sprinkle the mixture evenly over the apples.
Place the pan in the preheated oven and bake for about 1 hour, or until the topping is golden brown and the apples are tender. Allow the Apple Slab Pie to cool slightly before cutting it into squares and serving.
