Preheat the oven to 400°F. Lightly grease a standard muffin tin, or apply paper/silicone liners to the cups.
Make the topping. In a medium bowl, combine the ½ white whole wheat flour, cane sugar, brown sugar, oats, cinnamon, sea salt, and vegan butter. Using a spoon, stir everything together to get the vegan butter pieces coated in the flour mixture. Then, using your hands, start combining the mixture by pressing the pieces of butter between your fingers. Keep going until you have a slightly wet sand-like texture and a uniform, crumbly consistency. Set the topping aside in the fridge.
Place a small sauté pan over medium heat. Add the tablespoon of vegan butter to the pan and swirl it around. Add the chopped apples to the pan along with ½ teaspoon of ground cinnamon. Stir to combine. Keep sautéing the apples until they are slightly softened, about 3 minutes. Take the apples off the heat and allow them to cool slightly.
In a small bowl, combine the non-dairy milk, apple cider vinegar, and ground flax seeds. Stir to combine and set aside for 5 minutes.
In a large bowl, combine white whole wheat flour, brown sugar, baking powder, baking soda, sea salt, nutmeg, and the remaining 1 teaspoon of ground cinnamon. Stir to combine.
In the small bowl with the non-dairy milk and flax mixture, whisk in the sunflower oil, applesauce, vanilla, and lemon zest. Once combined, add that mixture to the large bowl with the flour mixture. Add about ⅔ of the sautéed apples to the bowl as well. Using a rubber spatula, stir everything together until JUST combined and there are no large pockets of dry ingredients.
Using a ⅓ cup measure, portion out the muffin batter evenly into all of the muffin cups. Top the portions of batter with the remaining sautéed apples. Retrieve the oat topping from the fridge and evenly distribute it amongst the tops of the muffins. This is a fair amount of topping, so you’ll have to push it into the tops of the muffins.
Bake the muffins until the tops are golden brown and a paring knife inserted into the center of one comes out clean, about 20-25 minutes. Let the muffins cool completely in the pan before enjoying.