Make the fruit filling: In a large bowl, combine sliced peaches, blueberries, white sugar, cinnamon, nutmeg, lemon juice, and cornstarch. Toss everything gently to coat the fruit evenly.
Add to baking dish: Pour the fruit mixture into a 2-quart baking dish and spread it out. Bake for 10 minutes to begin softening the fruit.
Prepare the topping: In a separate bowl, mix flour, white sugar, brown sugar, baking powder, and salt. Grate the cold butter into the mixture using a box grater, then cut it in with your fingertips or a pastry cutter until it resembles coarse crumbs. Stir in boiling water just until combined.
Top the fruit: Remove the dish from the oven and drop spoonfuls of the cobbler topping evenly over the warm fruit mixture.
Sprinkle cinnamon sugar: Mix white sugar and cinnamon in a small bowl, then sprinkle over the topping.
Bake again: Return to the oven and bake for about 30 minutes, until the topping is golden brown and the fruit is bubbling.
Let cool slightly, then serve warm—delicious on its own or with a scoop of vanilla ice cream.
Video
Notes
Store any leftovers covered in the fridge for up to 4 days. Reheat in the microwave or in a 350°F oven until warm. Freezing isn’t recommended, as it can affect the texture of the fruit and topping.