01 -
Preheat oven to 425°F. Place a large baking sheet covered in foil on the lowest rack in the oven to catch any drips during baking.
02 -
In a large bowl, combine the peaches, blueberries, and lemon juice. Sprinkle with white sugar, brown sugar, cinnamon, and cornstarch. Stir to coat and pour into a 9×13-inch baking dish.
03 -
Bake the fruit filling in the preheated oven for 10 minutes.
04 -
While the filling is baking, combine the flour, white sugar, brown sugar, baking powder, and kosher salt in a bowl. Whisk to combine. Add the grated butter and toss to mix. Stir in boiling water just until combined, leaving lumps of butter in the mixture.
05 -
Remove the peaches from the oven and drop spoonfuls of topping over the fruit filling. Use the smallest cookie scoop to make even mounds.
06 -
Sprinkle the cobbler topping with the cinnamon sugar mixture.
07 -
Bake the cobbler until the crust is golden and a toothpick inserted into the crust comes out clean, about 28 minutes. Serve warm or at room temperature.