Which Apples Work Best?
Choose apples that keep some shape when baked. Granny Smith brings tartness; Honeycrisp, Braeburn, Pink Lady, or other firm eating apples bring sweetness and flavor. A mix is particularly good. Using all very soft apples can produce a filling that breaks down before the pastry finishes baking.
Cut the apples into pieces of similar thickness so they cook evenly. Slices about ¼ inch (6 mm) thick work well in this shallow pie. Thick chunks can remain firm; paper-thin slices can turn mushy.
Why Use Cornstarch?
Apples release juice as they bake. Cornstarch thickens that juice into a soft filling that stays with the fruit when you cut the pie. It also helps prevent liquid from running across the pan.
Measure it carefully and toss it thoroughly with the apples. Pockets of dry starch are unpleasant, while too little can leave the filling loose. The filling must bubble during baking for the juices to thicken properly.
Why Keep the Butter and Water Cold?
Cold pieces of butter remain distinct in the dough. As the pastry bakes, those pieces help create a flaky texture. Warm butter blends too completely into the flour, giving you a denser crust.
Keep the water cold as well, and add it gradually. The dough needs enough moisture to hold together when squeezed, but it should never feel wet or sticky.
How to Make Apple Slab Pie
1. Make the Pastry Dough
In a large bowl, whisk the flour, sugar, and salt together. Add the cold butter. Use a pastry cutter or your fingertips to work it into the flour until you have a mixture of coarse crumbs with some pea-sized butter pieces still visible.
Drizzle in ½ cup (120 ml) of the cold water and toss with a fork. Add more water, a tablespoon at a time, until a handful of dough holds together when pressed. You may not need the full ¾ cup.
Turn the mixture onto a clean surface. Press it together gently; avoid kneading it until smooth. Divide it into two portions, making one slightly larger for the bottom crust. Flatten each into a rectangle, wrap, and refrigerate for at least 1 hour.
You can make the dough a day ahead. Keeping it rectangular now makes it easier to roll into the shape of the pan later.
2. Prepare the Apples
When you are ready to assemble, wash the apples under running water, even if you intend to peel them. The FDA recommends washing produce before peeling or cutting so material on the surface is less likely to be transferred by the knife. FDA
Peel and core the apples, then slice them about ¼ inch (6 mm) thick. Put them in a large bowl and toss with the brown sugar, granulated sugar, cornstarch, lemon juice, cinnamon, nutmeg, and salt.
Let the filling stand for 10 minutes while you roll the dough. Stir it again before adding it to the pan so the seasoning and starch are distributed evenly.
3. Prepare the Pan and Roll the Bottom Crust
Heat the oven to 400°F (200°C). Position a rack in the lower third of the oven. Lightly grease a 9-by-13-inch (23-by-33-cm) metal baking pan.
On a lightly floured surface, roll the larger dough portion into a rectangle about 13 by 17 inches (33 by 43 cm). The extra dough allows for the bottom and sides of the pan.
Carefully transfer the pastry to the pan. Ease it into the corners without stretching it, and leave some overhang around the edges. If it tears, press a small scrap of dough over the opening. Chill the lined pan while you roll the top crust.
4. Add the Apple Filling
Spoon the apples evenly into the crust. Include the seasoned juices and starch left in the bowl, but distribute them rather than pouring them all into one corner. Gently level the fruit so the top pastry has an even surface.
An apple slab pie is shallower than a deep-dish pie. The apples may look generously piled at this stage, but they soften and settle as they bake. Avoid pressing them down so hard that they damage the bottom pastry.
5. Add and Seal the Top Crust
Roll the second dough portion into a rectangle large enough to cover the pan. Lay it over the apples. Trim excessive dough, leaving enough around the edge to join the top and bottom crusts.
Pinch or crimp the edges to seal them. Cut several 1-inch (2.5-cm) slits across the top with a sharp knife so steam can escape. The slits also allow you to see whether the filling is bubbling near the end of baking.
Beat the egg with 1 tablespoon water. Brush a thin layer over the top crust and sprinkle with coarse sugar if using. Do not let a large puddle of egg wash collect around the edges.
If the assembled pie feels soft or the butter is beginning to warm, refrigerate it for 15 minutes before baking.
6. Bake Until the Crust and Filling Are Done
Place the pan on a larger rimmed baking sheet to catch any overflow. Bake at 400°F (200°C) for 20 minutes. Then reduce the oven temperature to 375°F (190°C) and bake for another 30–40 minutes.
The apple slab pie is ready when the pastry is golden brown and the filling visibly bubbles through several vents. Check a middle vent, not just an edge: bubbling at the center is a better sign that the juices have heated through and thickened.
If the crust browns too quickly, loosely cover the top with foil for the remaining baking time. If the top is golden but the base still looks pale, move the pan slightly lower in the oven and give it more time, watching carefully.
7. Cool Before Slicing
Set the pan on a wire rack and cool for at least 2 hours. This rest allows the filling to settle and gives the thickened apple juices time to hold together.
Cut into 12 rectangles for a dessert serving or 16 squares for a buffet. A sharp knife makes the cleanest cuts. Lift the first piece carefully with a thin spatula; the rest will be easier once you have room to work.
Serve the apple slab pie as it is, or warm individual slices after cutting and add vanilla ice cream.