Preheat oven to 350ºF (175ºC) and line a muffin tin with muffin liners.

Prepare the streusel topping: In a bowl, combine old-fashioned oats, all-purpose flour, light brown sugar, chopped walnuts, cold unsalted butter, and vanilla extract. Use your fingers or a pastry blender to cut the ingredients together until a crumbly texture forms. Set aside.

In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and ground cinnamon. Set aside.

In a large bowl, cream together the light brown sugar, vanilla extract, and melted unsalted butter until well combined.

Add the unsweetened applesauce to the creamed mixture and mix until combined. Beat in the eggs one at a time, then stir in the almond milk.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Gently fold in the diced apples.

Fill each muffin cup about 3/4 full with batter. Sprinkle 1-1.5 tablespoons of the streusel topping over each muffin. Gently press the streusel into the top of the batter.

Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.