Preheat your oven to 425 degrees F and line a standard muffin pan with paper liners. Spray lightly with oil.
In a small bowl, stir the ground flaxseeds and water together. Set aside to thicken; these are your flax eggs.
In a large bowl, whisk together all the dry ingredients: flour, sugars, baking powder, cinnamon and salt.
To the bowl with the dry ingredients, pour in the melted vegan butter, oat milk, vanilla and flax eggs. Stir with a large spoon until just combined, but do not over mix or your muffins will be dense.
Fold in the apples.
Using a small measuring cup or ice cream scoop, transfer the batter to the prepared liners and fill to the top.
Make the crumb topping: In a small bowl, combine all ingredients and mix with a fork until crumbly.
Sprinkle the crumb all over the tops of the muffin batter.
Bake for 5 minutes at 425 degrees, then reduce heat to 350 degrees and bake for 15-18 more minutes, until a toothpick inserted in the center comes out clean. This will help the muffins be tall!
Make the icing: Add the powdered sugar, vanilla and non-dairy milk to a small bowl and use a fork to whisk until combined and smooth.
Drizzle icing over warm or cooled muffins. They will stay fresh for 2-3 days at room temperature, up to 1 week in the refrigerator and can also be frozen.