Preheat oven to 350 degrees F. Place the first three ingredients in a large bowl and whisk to combine. Mix in the nectarines and 1/3 cup brown sugar. Spoon filling into a 9" x 13" baking dish. Place the remaining ingredients in a bowl and work with your fingertips until crumbly. Sprinkle over the filling. Bake the crumble until golden brown and filling is thickened and bubbling, about 50 to 55 minutes. Serve warm or at room temperature.