Preheat an oven to 425 degrees F (220 degrees C).
In a bowl, beat together the flour, granulated sugar, milk, and eggs until thoroughly blended. Set aside.
In a 10 to 12-inch (25 to 30-cm) heatproof frying pan over medium-high heat, melt the butter. Add the peaches or nectarines and cook, stirring often, until the fruit is hot and slightly softened, 3 to 5 minutes, depending on ripeness. Remove from the heat and scatter with the berries.
Pour the egg mixture over the fruit in the pan and immediately put in the oven. Bake until puffed and golden, about 18 minutes.
Remove the pancake from the oven and sift confectioners' sugar over the top. Cut into wedges to serve. If desired, offer lemon wedges on the side.