Preheat your oven to 375°F (190°C). Lightly grease an 8x8 inch baking dish or a similar sized pie dish.

Prepare the filling: In a large bowl, combine the sliced nectarines, granulated sugar, cornstarch, lemon juice, and 0.5 teaspoon of cinnamon. Toss gently until the nectarines are evenly coated. Pour the mixture into the prepared baking dish and spread into an even layer.

Prepare the streusel topping: In a separate medium bowl, whisk together the flour, rolled oats, brown sugar, chopped pecans, 1 teaspoon of cinnamon, and salt.

Cut in the butter: Using a pastry blender or your fingertips, cut the cold, cubed butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized clumps remaining.

Assemble and bake: Sprinkle the streusel topping evenly over the nectarine filling. Place the dish on a baking sheet (to catch any potential drips) and bake for 35 to 40 minutes, or until the topping is golden brown and the filling is bubbly.

Let the crisp cool for at least 15 minutes before serving to allow the juices to set slightly. Serve warm, perhaps with a scoop of vanilla ice cream.