Preheat oven to 350F and grease a 9 x 13 inch baking dish.
In a large bowl, whisk together the oats, almond flour, pecans, brown sugar, cornstarch, cinnamon, baking powder and salt.
Add the melted butter and vanilla and mix together until well combined. Place in fridge while you make the apple filling.
In a large bowl, combine the chopped apples, brown sugar, cornstarch, lemon juice, vanilla, cinnamon, nutmeg and salt. Stir until the apples are well coated.
Pour the apple mixture into the bottom of the prepared baking dish.
Spread the oat topping over the apples in an even layer.
Bake for 20-25 minutes, or until the topping is golden brown. Remove from oven and carefully cover the crisp with foil. Bake for another 15-20 minutes, or until the apple filling is bubbling around the edges.
Let cool for 15-20 minutes before serving.
Notes
I used Gala apples for this recipe as they're sweet and hold their shape well. They're also more affordable and readily available compared to some other varieties. If you prefer a more tart/sweet apple, Honeycrisp works well, and for even more tartness, use Granny Smith. If you use these varieties, you may wish to add a little more sugar to the filling.
Cut the apples into even sized pieces so they cook evenly. I like big chunks of apples in my crisp, so I cut them into 1-inch pieces. If you cut the apples smaller, they will cook quicker so you'll need to reduce the baking time.
The crisp topping browns quickly and the pecans burn easily, so you'll want to cover the crisp with foil midway through baking.
To keep this crisp gluten-free, make sure you use certified gluten-free oats.