Preheat the oven to 375℉. Have a 9x13-inch baking pan ready.
To prepare the topping: In a medium mixing bowl, stir together the 2 cups old-fashioned oats, 1 cup oat flour, ⅔ cup brown sugar, 1 teaspoon ground ginger, and ½ teaspoon fine sea salt . Mix in the melted 8 tablespoons unsalted butter and stir until the mixture is moistened and well combined. Place the bowl in the fridge to chill while you make the filling and the oven heats.
To prepare the filling: In a large bowl, mix together the 5 pounds ripe nectarines, ⅔ cup brown sugar, 4 tablespoons arrowroot starch, and 1 teaspoon ground cinnamon. Sprinkle in the 2 stalks rosemary if using.
Spread the filling in an even layer in the baking pan. Remove the topping from the fridge (break it up a bit if necessary) and distribute it evenly over the top of the fruit.
Place in the oven and bake for 40-45 minutes, until the filling is bubbling around the edges and the top is lightly golden. Remove from the oven and let rest for at least 15 minutes before serving.
Notes
Chill the topping - Make the topping first, then pop the bowl in the fridge while you prepare the filling. If the topping is too firm when you take it out of the fridge, break it up with your hands before scattering it over the filling.
Let it rest - After removing the crisp from the oven, let it sit for at least 15 minutes so that the filling sets up a bit.
Add other fruit - Pair the nectarines with peaches for a stone fruit crisp. I also love a blueberry-nectarine crisp (swap 1 cup of blueberries for 1 peach) or mix in some blackberries.
Natural sweetener option - I like to use honey in the filling instead of brown sugar for a natural sweetener option (also try maple syrup). To do this, just substitute ⅔ cup of honey for the brown sugar. Then use coconut sugar in the topping.
Oat and oat flour substitute - Substitute almond flour and sliced almonds for the oat flour and oats, or use 1 ½ cups of your favorite 1:1 gluten-free flour blend for the oat flour.
Make it dairy-free - Substitute your favorite non-dairy butter or melt coconut oil for the butter.