Preheat your oven to 375°F (190°C). Position a rack in the middle.

In a large bowl, combine the sliced peaches, 1/2 cup granulated sugar, brown sugar, cornstarch, lemon juice, cinnamon, nutmeg, and a pinch of salt. Stir gently until the peaches are evenly coated. Let the mixture sit for 10-15 minutes while you prepare the topping.

In a separate medium bowl, whisk together the flour, 1/2 cup granulated sugar, baking powder, baking soda, and 1/2 teaspoon salt.

Add the cold butter cubes to the flour mixture. Using a pastry blender or two forks, cut the butter into the flour until the mixture resembles coarse meal with pea-sized butter pieces. Do not overwork — the butter should remain cold.

Pour the cold buttermilk into the flour-butter mixture. Stir with a fork just until a shaggy dough forms. Do not overmix; the dough will be sticky.

Pour the peach mixture into the ungreased 9x13-inch baking dish and spread into an even layer.

Drop spoonfuls of the biscuit dough over the peaches, spacing them about 1 inch apart. The dough will spread as it bakes. Sprinkle the top with turbinado sugar for crunch.

Place the dish on a baking sheet to catch any drips. Bake for 40-45 minutes, until the fruit is bubbling around the edges and the biscuit topping is deep golden brown. If the topping browns too quickly, tent loosely with foil after 30 minutes.

Let the cobbler cool on a wire rack for at least 15 minutes before serving. This allows the juices to thicken. Serve warm, with vanilla ice cream or whipped cream if desired.