In a mixing bowl, combine sliced nectarines, granulated sugar, and ground cinnamon. Toss to coat evenly.
Transfer the nectarine mixture into a greased baking dish, spreading it out evenly.
In another bowl, prepare the crumble topping by mixing flour, brown sugar, baking powder, and salt. Add the cold, cubed butter and use a pastry cutter or fingers to blend until the mixture resembles coarse crumbs.
Sprinkle the crumble topping evenly over the nectarines in the baking dish.
Bake in the preheated oven for 30 minutes, or until the topping is golden brown and the filling is bubbly.
Remove from oven and let it cool slightly before serving. Enjoy warm, optionally with a scoop of vanilla ice cream.
Notes
For an added twist, consider adding a handful of chopped nuts to the crumble topping or a splash of vanilla extract to the filling mixture.