Bring a large pot of water to a boil. Cut a small "X" on the bottom of each peach, then gently lower them into boiling water for 30 seconds. Transfer immediately to an ice bath. The skins should slip off easily with your fingers or a paring knife. Slice peaches into 1/2-inch wedges, discarding pits.
In a large bowl, combine sliced peaches, both sugars, cinnamon, nutmeg, lemon juice, and cornstarch. Toss gently until peaches are evenly coated with the sweet spice mixture.
Preheat your oven to 375°F (190°C). While it's heating, butter an 8x8-inch baking dish or 9-inch cast-iron skillet.
Pour your prepared peach mixture into the buttered baking dish, ensuring an even distribution.
In a medium bowl, whisk together flour, sugar, baking powder, and salt. Using your fingers or a pastry cutter, work the cold butter cubes into the dry ingredients until the mixture resembles coarse crumbs.
Pour the boiling water into your flour mixture and stir just until combined. The batter will seem slightly thick and lumpy.
Drop spoonfuls of the batter over your peach filling, leaving some spaces for the fruit to peek through. Sprinkle with a tablespoon of sugar for a sparkling finish.
Bake in your preheated oven for 40-45 minutes, until the crust is golden brown and the filling is thick and bubbling around the edges.
Allow to cool for 15-20 minutes before serving. Serve warm with a scoop of vanilla ice cream if desired.