Preheat oven to 350°F (175°C) and lightly grease a 9×9-inch baking dish.
In a large bowl, combine sliced nectarines, lemon juice, sugar, cornstarch, and vanilla extract.
Transfer the fruit mixture to the prepared baking dish.
In a separate bowl, mix oats, flour, brown sugar, cinnamon, and salt.
Stir in melted butter until crumbly.
Evenly sprinkle the topping over the nectarine filling.
Bake for 40-45 minutes or until golden brown and bubbly.
Allow the crisp to cool for 20 minutes before serving.
Serve warm with vanilla ice cream or whipped cream if desired.