Preheat and Prep: First, preheat your oven to 350°F (175°C). Get out your baking dish (a 9x9 square, 8x8, or a 9-inch round pie plate all work great).
Prepare the Apples: Peel, core, and slice your apples about 1/4-inch thick. You don't need to be perfect. Place all the apple slices into a large mixing bowl.
Season the Filling: Sprinkle the cinnamon, nutmeg (if using), and pinch of salt over the apples. Toss everything together with your hands or a spatula until the apples are evenly coated in spice.
Assemble the Base: Pour the seasoned apples into your baking pan and spread them into an even layer.
Make the Crumb Topping: In the same large bowl (no need to wash it!), add all the crumb topping ingredients: the GF flour, GF oats, brown sugar, softened butter, flaxseed meal, and 1/2 teaspoon of cinnamon.
Mix the Topping: Using a pastry cutter or just your hands, work the butter into the dry ingredients. Mix and squeeze the mixture until it's crumbly and holds together in small clumps when you press it. Mixing by hand is best to get those perfect, rustic crumbles.
Cover the Apples: "Drop" small pieces of the crumble mixture evenly over the top of the apples. Continue until the apples are completely covered with that delicious topping.
Bake to Perfection: Bake at 350°F for 40-50 minutes. You'll know it's done when the topping is a beautiful golden brown and the apple filling is bubbling around the edges.