Preheat the oven to 360°F (convection if available). Lightly grease a 9×13-inch baking dish.
In a large bowl, toss the berries with sugar, flour, lemon juice, and a pinch of ground cinnamon. Let rest 10 minutes so juices develop.
Transfer the berry mixture to the prepared pan. Bake for 10 minutes to slightly set the filling.
While the fruit bakes, whisk together flour, sugar, baking powder, and salt in a medium bowl.
In a separate vessel, stir the melted butter into cold milk until little butter clumps form. Gently fold into dry ingredients until just combined.
Spoon dollops of the topping over warm berries, spreading loosely. Sprinkle with coarse sugar.
Return to oven and bake 40–45 minutes, or until the crust is golden and the center bubbles vigorously. If the crust darkens too fast, tent with foil.
Let rest 15 minutes before serving so the filling thickens further. Serve warm with vanilla ice cream.
