Stir together flax meal and water and set aside to thicken for 10-15 minutes while preparing the rest of the ingredients.
Add oil, applesauce, milk, vanilla, apple cider vinegar and both sugars to a large mixing bowl. Use a hand mixer or whisk to beat until well combined.
In another bowl, stir together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Stir the flax eggs into the wet ingredients. Add the dry mixture to the wet mixture and mix until well combined.
Fold in the apples and walnuts to the batter.
Scoop muffin batter evenly into each liner, filling just over ¾ of the liner.
Make the topping in a small bowl by stirring together oats, brown sugar, melted butter and cinnamon.
Top each muffin with an even amount of oat topping. Bake for 25-27 minutes, or until a toothpick comes out clean.
Gently press the streusel into the muffins