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, gently toss the sliced nectarines with the granulated sugar, cornstarch, and lemon juice until evenly coated. Pour the mixture evenly into the prepared baking dish.

Prepare the topping: In a separate medium bowl, combine the flour, brown sugar, cinnamon, and salt. Whisk briefly to break up any sugar clumps.

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

Assemble and bake: Sprinkle the crumb topping evenly over the nectarine filling. Place the dish on a baking sheet (to catch any drips) and bake for 35 to 45 minutes, or until the topping is golden brown and the filling is bubbly around the edges.

Cool slightly before serving warm. This crisp is excellent served à la mode with vanilla ice cream.