To prepare the base: Put the flour in a big, wide bowl. Mix the sugar and salt. Add the pieces of butter and cover them with the flour. With the first 3 fingers of each hand, break the butter into small pieces slightly larger than peas. You can also use two knives or a sturdy pastry blender to do this. I like to use my hands.
When the mixture looks mostly the same, a bit crumbly, and the butter is broken into small pieces, add ½ cup of water using a rubber spatula. Mix until the water is absorbed and the dough begins to stick together. If the dough is still dry and crumbly, add more water, little by little, until the dough begins to stick together. Use your hands to mix the dough until it becomes a soft and slightly loose ball. Cut the pie dough into two pieces, one a little bigger than the other. You will need the bigger piece for the bottom part of the crust. Place one portion on a piece of plastic wrap and shape it into a rectangle that is about 1 inch thick. Use the plastic wrap to help the dough stick together, if necessary. Do the same with the rest of the dough. Cool the dough in the fridge for at least 2 hours before using it.
To prepare the filling: In a big bowl, mix the apple pieces with the lemon juice. Combine the sugar, cornstarch, cinnamon, nutmeg, allspice, and salt in a medium bowl. Sprinkle the sugar and spice mix over the apples and mix well to cover them; then, leave them to the side.
To put together the slab pie: Heat the oven to 375 degrees Fahrenheit. Place a piece of parchment paper at the bottom of a baking sheet that measures 10 by 15 inches and is 1 inch deep. If your pan is a bit smaller than this one (like mine was), don’t worry – this recipe will still turn out well; you’ll just have some extra dough left over.
Flatten the big piece of dough on a surface sprinkled with flour using a rolling pin until it forms a rectangle measuring 18×13 inches. Put the dough in the pan and fold the edges over the sides. Gently press the dough into the inside and corners of the pan. Put the pan in the fridge or freezer while you flatten the second piece of dough into a 16×11-inch rectangle (make sure to keep the dough in the pan as cold as you can).
Take the pan out of the fridge or freezer. Pour the apples and their juices onto the dough in the pan. Even though it might look like a lot of apples, they will shrink as they cook. Put the second piece of dough on top of the apples. Cut the dough so that both the top and bottom edges extend about ¾-inch beyond the pan. Close the edges firmly using your fingers.
Apply heavy cream or egg wash on the top of the dough. With a sharp knife, make many 1-inch cuts on the top of the dough to let the steam out while baking. Put the entire sheet on a bigger baking tray – some things might spill, and it’s better to catch them on the larger tray than on the bottom of your oven.
Bake for 40 to 45 minutes until the top is a nice golden brown and the filling is bubbling through the holes in the crust. Let the pan cool on a metal rack for at least 45 minutes before cutting and serving.
