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, ½ cup sliced almonds if using, 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 peaches, ⅔ cup brown sugar, 4 tablespoons arrowroot starch, 2 teaspoons vanilla extract, and 1 teaspoon ground cinnamon.
Spread the peaches 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 peaches.
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
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 - Throw in some berries or pair the peaches with nectarines. I love a peach-blueberry crisp (swap 1 cup of blueberries for 1 peach), and peach-raspberry is another winning combo.
Sweeten with honey - I'll often use honey in the filling instead of brown sugar. I love how honey's intense floral sweetness plays off of the peaches. To do this, just substitute ⅔ cup of honey for the brown sugar.
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 for the butter or melted coconut oil for the butter.