How to Make It
Cut the Grannysmith apple and 1 Honeycrisp apple into a small dice (about 2½ cups) and place in a medium-sized bowl. Add 3 tablespoons brown sugar, granulated sugar, salt, cinnamon, and nutmeg, stir to combine. Stir in the lemon juice.
In a small bowl, combine the oats, pecans, remaining tablespoon brown sugar, and flour. Add the pieces of butter and incorporate them with your hands until a crumble forms. Gently fold the crumble into the apple mixture until just incorporated.
Slice off the very tops of the remaining 7 Honeycrisp apples and, using a melon baller (or spoon and a paring knife), hollow out the insides of the apples, being sure to remove the core and seeds. Using the very tip of the paring knife, cut a horizontal slit around the circumference of the apple, just barely piercing the skin, being careful not to slice through the apple’s wall (this acts as a steam vent for the apples while baking). Divide the apple filling evenly amongst the apples.
Preheat oven to 375° F. Remove the piecrust from the refrigerator and cut into small strips (just longer than the width of each apple). Weave the strips into a lattice pattern atop each apple. Place the apples in a baking dish.
Whisk the egg in a small bowl and brush the egg wash onto each apple’s lattice. Sprinkle with turbinado sugar. Pour 1 inch of water into the baking dish, then cover the dish with foil, ensuring the foil isn’t touching the tops of the apples. Bake for 20 minutes. Remove the foil and bake an additional 20-25 minutes, until the crust is golden brown and the apples have softened.