Preheat oven to 350 degrees.
Line 9 in x 13 in pan with parchment paper or cooking spray.
Mix flour, sugar and salt together in a medium bowl.
Cut in the butter until it resembles coarse crumbs.
Press into the bottom of the pan.
Bake 12-15 minutes until lightly browned, while crust is in the oven, prepare filling.
Place all ingredients into small sauce pan on medium heat. Bring to a simmer and continue to cook 10-15 minutes until jam like consistency. Stir occasionally to prevent scorching.
Set aside and let cool (filling will thicken as it cools); while filling is cooking, prepare the crumb topping.
Combine butter, flour, sugar and cinnamon with your fingers until crumbly.
Spoon nectarines over crust and gently spread evenly.
Top with crumb topping.
Bake 20-25 minutes, until fruit is bubbling and topping is lightly browned. Allow to cool before slicing into bars.