Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with parchment paper liners. Silicone liners work exceptionally well with paleo batters, as they prevent sticking better than traditional paper liners.
In a large bowl, whisk together the almond flour, coconut flour, baking soda, cinnamon, nutmeg, and salt. Break up any clumps in the almond flour with your fingertips for the smoothest batter possible.
In a separate medium bowl, whisk the eggs until frothy (about 30 seconds of vigorous whisking), then add the maple syrup, melted coconut oil, vanilla extract, and apple cider vinegar.
Slowly pour the wet ingredients into the dry ingredients, gently folding with a spatula until just combined. Avoid overmixing - aim for about 10-12 gentle folds.
Add the diced apples and 3/4 cup of walnuts to the batter, gently folding to distribute evenly. For extra flavor dimension, reserve a few apple pieces to press into the tops of the muffins before baking.
If using the streusel topping, in a small bowl, combine all streusel ingredients and use your fingertips to create a crumbly texture.
Using an ice cream scoop or 1/4 cup measure, divide the batter evenly among the prepared muffin cups. They should be about 3/4 full to allow room for rising. Gently tap the pan on the counter twice to release any large air bubbles.
Sprinkle the streusel topping (if using) evenly over each muffin. Alternatively, simply sprinkle the tops with a few additional chopped walnuts and a light dusting of cinnamon.
Bake in the preheated oven for 22-25 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs. The tops should be golden brown and slightly springy to the touch.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely. This resting period allows the crumb structure to set, ensuring your muffins maintain their shape.