Preheat the oven to 350°F (175°C). Spray a muffin tin with cooking spray or line it with muffin cups.
Prepare the flax egg: combine ground flax with water. Whisk together and allow to sit for five minutes until it gels together.
Whisk together the flour, brown sugar, baking powder, baking soda, cinnamon, and salt in a large mixing bowl.
Whisk together the flax egg, oil, almond milk, and vanilla extract in a medium mixing bowl.
Make a well in the bowl of dry ingredients. Add the wet ingredients and fold them until just combined.
Add the chopped apples and gently fold them into the batter. Allow the batter to rest for 10 minutes.
While the batter is resting, prepare the streusel topping. Mix the oats, flour, brown sugar, and cinnamon in a small bowl. Add the cubed vegan butter and use a fork or your fingers to combine until a crumbly texture is achieved.
Divide the muffin batter into the prepared muffin tin. You will have enough batter for about nine muffins. Evenly sprinkle the streusel topping onto each muffin.
Bake for 20-25 minutes or until a toothpick inserted into the center of the muffins comes out clean.
Allow the muffins to cool in the muffin tin. Enjoy!