Preheat oven to 375 degrees. Generously butter bottom and sides of an 8- by 8-inch baking dish.
In a large bowl, combine plums, nectarines, brown sugar, cinnamon and 1 tablespoon flour. Transfer mixture to prepared baking dish.
In a medium bowl, combine remaining 1 cup flour, granulated sugar, baking powder, salt, nutmeg and egg and stir until crumbly. Sprinkle flour-sugar mixture over fruit. Drizzle with melted butter. Bake until top is brown, about 40 minutes. Remove from oven to a wire rack. Serve warm or at room temperature.
Nutrition information per serving of 8: