Preheat the oven to 350 degrees Fahrenheit. Line a 12-tin muffin tray with paper liners.
In a small bowl, combine crumb topping ingredients with your fingers so that the coconut oil breaks down into pea-sized or smaller pieces. Start with 4 tablespoons sugar, add a little more if needed. Set aside.
In a blender, add chia seeds, hot water, lemon juice, applesauce, and coconut oil. Set aside.
In a large bowl, add almond flour, oat flour, arrowroot starch, baking soda, baking powder, ground cinnamon, salt, and coconut sugar. Whisk to combine.
Add vanilla extract to the mixture in the blender, and blend on high until smooth. Fold liquid mixture into the dry mixture. Once evenly combined, fold in diced apples.
Evenly divide muffin batter amongst muffin tins. Sprinkle crumb topping on top of each. Bake for 25-28 minutes, or until a toothpick comes out clean. Remove gluten-free apple crumb muffins from oven and place tray on a wire cooling rack. Allow to cool. Store in an airtight container.