Preheat your oven to 375°F. Place an oven rack in the center position. Butter a 9x13-inch baking dish thoroughly and set aside. Checkpoint: Your oven should be at temperature and your dish ready to fill.

Peel 6 medium Granny Smith apples and 3 medium Honeycrisp apples using a vegetable peeler. Cut each apple in half, remove the core with a small knife, then slice into 1/4-inch-thick slices. You should have about 10 cups of sliced apples. Checkpoint: All apples should be peeled, cored, and sliced uniformly.

In a large mixing bowl, add the sliced apples. Sprinkle 1/2 cup granulated sugar, 1/4 cup brown sugar, 2 tablespoons all-purpose flour, and 1 tablespoon cornstarch over the apples. Add 1 teaspoon vanilla extract, 1/2 teaspoon cinnamon, 1/4 teaspoon nutmeg, 1/4 teaspoon ginger, 1/8 teaspoon allspice, and 1/4 teaspoon salt.

Using a large spoon or silicone spatula, gently toss the apples and sugar mixture together for 2–3 minutes until all apples are evenly coated and the sugar begins to dissolve. Pour in 1 tablespoon fresh lemon juice and stir for 30 seconds. Checkpoint: Apples should be evenly colored and glistening; liquid should be pooling at the bottom of the bowl.

Let the apple mixture sit undisturbed for 5 minutes at room temperature to allow the juices to release and draw out. Checkpoint: You should see obvious liquid in the bottom of the bowl.

Pour the entire apple mixture (including all liquid) into your prepared 9x13-inch baking dish, spreading the apples in an even layer. Cut 2 tablespoons of cold unsalted butter into small pea-sized pieces and scatter them over the apple filling. Checkpoint: Apples should be level, and butter pieces should be distributed across the surface.

In a medium mixing bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt for 30 seconds until combined. Checkpoint: The mixture should look pale and uniform with no lumps.

Cut 6 tablespoons of cold unsalted butter into 1/2-inch cubes and add to the flour mixture. Using a pastry cutter, two forks, or your fingertips, work the cold butter into the flour until the mixture resembles coarse breadcrumbs or sand—some pea-sized pieces of butter should remain visible. This should take 2–3 minutes. Checkpoint: The texture should feel crumbly and cool, not smooth or clumpy.

In a small bowl, crack 1 large egg and whisk it with 1/2 cup sour cream and 1 teaspoon vanilla extract for 1 minute until smooth and light. Checkpoint: The egg-sour cream mixture should be smooth and uniform, no lumps of egg white.

Pour the wet mixture into the center of the flour-butter mixture. Using a rubber spatula or wooden spoon, gently fold the ingredients together for 1–2 minutes until just barely combined—some streaks of flour should still be visible. Do not overmix. Checkpoint: The dough should be shaggy and slightly lumpy, not smooth.

Fold 1 cup lightly chopped pecan halves into the dough with 3–4 gentle strokes. Checkpoint: Pecans should be evenly distributed throughout but the dough should still be lumpy.

Drop rounded 2-tablespoon spoonfuls of the biscuit batter over the apples, spacing them about 2 inches apart and leaving some gaps showing the apple filling—this is a rustic cobbler, so don't worry about coverage being perfect. You should have 12–15 drops of batter. Checkpoint: Apples are visible between biscuit drops; batter is in separate mounds, not spread flat.

In a small bowl, mix together 2 tablespoons brown sugar, 1 tablespoon granulated sugar, and 1/4 teaspoon ground cinnamon. Sprinkle this mixture evenly over the biscuit topping. Checkpoint: The topping should have a visible sugar-spice coating.

Place the baking dish on a baking sheet (to catch any drips) and transfer to your preheated 375°F oven on the center rack. Bake for 40–45 minutes until the biscuit topping is golden brown (not pale) and the apple filling is bubbling visibly around the edges—you should see hot, glossy apple juice coming up the sides. Checkpoint: The biscuits should be deep golden brown, not light tan; apple filling should bubble around edges.

Remove the cobbler from the oven using oven mitts. Set the baking dish on a heat-safe surface or trivet. Let cool undisturbed for 10–15 minutes—this allows the filling to set slightly and makes serving easier. Checkpoint: The cobbler should still be hot and fragrant but stable enough to scoop.

Serve warm directly from the baking dish using a spoon or ice cream scoop. Top each serving with vanilla ice cream or a dollop of whipped cream if desired.