Preheat the oven to 350ºF. Butter a shallow 8-by-11-inch (2-quart) baking pan.
Cream the brown sugar and butter together.
Sift the flour, baking powder, cinnamon, and salt together and beat them into the sugar and butter mixture with an electric mixer. The result should be crumbly, like streusel.
Press 1¼ cups of the mixture into the bottom of the prepared pan to make a crust.
Mix the blueberries, peaches, sugar, and nutmeg in a large bowl.
Add the remaining 1¼ cups of the streusel mixture and stir until the fruit is coated.
Spoon it over the crust as evenly as possible.
Bake at 350ºF for approximately 45 minutes.
Cool slightly before cutting into squares.
