Preheat your oven to 375°F (190°C). Grease a 10-inch cast iron skillet or a 9x13-inch baking dish with butter.

In a large bowl, combine the sliced peaches, brown sugar, granulated sugar, lemon juice, vanilla extract, cinnamon, nutmeg, and cornstarch. Toss gently to coat all the peach slices evenly.

Let the peach mixture sit for 10 minutes to macerate and release their juices. Stir once more, then transfer the filling into your prepared skillet or baking dish, spreading into an even layer. Dot the top of the filling with the small pieces of butter.

In a large bowl, whisk together the flour, granulated sugar, baking powder, salt, and cinnamon.

Add the cold butter cubes to the flour mixture. Using your fingertips or a pastry cutter, work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining. Work quickly so the butter stays cold.

Drizzle in the heavy cream and vanilla extract. Stir with a fork just until the dough comes together into a shaggy, slightly sticky mass. Do not overwork — lumpy is perfectly fine.

Drop large spoonfuls of the biscuit dough over the peach filling, distributing them somewhat evenly but leaving gaps for the filling to bubble up through. You should have about 8-10 rough mounds.

Brush the tops of the biscuit drops with the remaining 2 tablespoons of heavy cream. Sprinkle generously with turbinado sugar and a pinch of flaky sea salt if using.

Bake at 375°F for 40-45 minutes, until the biscuit topping is deep golden brown and cooked through, and the peach filling is bubbling vigorously around the edges. If the biscuit tops brown too quickly, tent loosely with foil for the last 10 minutes.

Remove from the oven and let rest for at least 10-15 minutes before serving. The filling will thicken as it cools slightly. Serve warm with vanilla ice cream or whipped cream.