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

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

Prepare the topping: In a separate medium bowl, whisk together the oats, 1/2 cup flour, brown sugar, 1/2 teaspoon cinnamon, and salt.

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

Assemble and bake: Sprinkle the crisp topping evenly over the nectarine filling. Place the dish on a baking sheet (to catch any drips) and bake for 40 to 45 minutes, or until the topping is golden brown and the filling is bubbly and tender.

Let cool for at least 15 minutes before serving warm, preferably with a scoop of vanilla ice cream.