Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Prepare the filling: In a large bowl, gently toss the sliced nectarines with 1/2 cup sugar, cornstarch, lemon juice, and cinnamon until evenly coated. Pour the fruit mixture into the prepared baking dish.

Prepare the topping: In a separate medium bowl, whisk together the flour, 1/3 cup sugar, baking powder, and salt.

Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Stir in the milk until just combined, being careful not to overmix the dough.

Drop large spoonfuls of the topping evenly over the fruit mixture in the baking dish, creating dollops or patches that cover most of the fruit. It does not need to be perfectly uniform.

Bake for 35 to 45 minutes, or until the topping is golden brown and the fruit filling is bubbling thickly around the edges. Allow to cool slightly for at least 15 minutes before serving warm.