Preheat the oven to 425°F. You will need a 10x15-inch sized standard jelly roll pan.
Remove your box of Pillsbury pie crusts from the refrigerator and let them come to room temperature, according to the package directions, before unrolling each crust. Once they can each be unrolled, you will lay the crusts, slightly overlapping in the center, onto your jelly roll pan so that you have about 2 to 3 inches or so of excess dough hanging off the shorter sides of the pan.
With a sharp knife, you will need to trim off the larger amount of excess dough hanging off the shorter ends of the pan.
You will need to place those two pieces of excess dough, one on each of the longer sides of the pan so that you fill in the gaps on the pan where the dough does not cover. You will want to face the rounded edge towards the center of the pan, and the straight edges will fill in the edges of the crust.
With your fingers, gently press the dough to seal all the seams together, tuck the border edges of the dough slightly under so that you have an even edge all around your pan. Crimp the outer crust the same as you would for a standard round pie. Set aside.
In a large mixing bowl, add the McIntosh apple slices, light brown sugar, cornstarch, melted unsalted butter, apple pie spice blend, fresh lemon zest, vanilla extract, and salt. Stir to evenly coat all the apple slices with the other ingredients.
Spread your coated apple slices in an even layer onto the prepared pie crust.
Bake for 25 to 30 minutes. Your crust should be lightly golden, your apple slab pie filling should be slightly bubbly, and your apple slices fork tender.
Allow your apple slab pie to cool on the counter for 30 minutes before adding the glaze.
To make the glaze whisk together the powdered sugar, unsalted butter, fresh lemon juice, heavy cream, vanilla extract, and salt. Whisk until smooth. You may need to add a little more heavy cream to thin out your glaze enough to be thick yet pourable.
Drizzle your glaze over your apple slab pie and allow your pie to continue to cool completely on the counter for another 30 to 45 minutes to allow your apple pie filling and glaze to firm up before slicing and serving.