Preheat your oven to 350 degrees.
Cube your unsalted butter and place in a bowl. Cover with plastic wrap and place in the freezer while you prep the rest of your ingredients.
Slice your nectarines into 1/4" thickness, leaving the skin on.
In a mixing bowl, combine the nectarine slices with 1/3 cup granulated sugar, lemon zest, lemon juice, vanilla extract, and cornstarch. Gently stir until the fruit is coated in the mixture and there's no excess mixture in the bottom of the bowl.
Transfer fruit to a well buttered 9x13" baking dish or enamel braiser.
In a large bowl add the flour, brown sugar, cinnamon, almonds, salt, old fashioned rolled oats and stir to combine. Add in the cubed butter. Using your fingers, pinch the butter into the oat mix (using a pastry cutter helps) until the butter resembles the size of peas.
Evenly scatter the crumble mix over the nectarines. Bake at 350 degrees, uncovered, for 35-40 minutes, or until juices are bubbling and crumble topping is golden brown. Let sit for 15 miuntes to cool before digging in.