Fill and bake the galette
In a bowl, lightly toss together the berries, lemon juice, sugar, and flour. Spoon the filling onto the dough, leaving a 2-inch (5-cm) border uncovered around the edge. Fold the edge up and over the filling, crimping to form loose pleats. Bake until the pastry is golden brown and the filling is bubbling, about 25 minutes. Transfer the galette to a wire rack and let cool slightly. Cut into wedges and serve warm. Top with whipped cream, if desired.