Mix together almond flour, baking powder, baking soda, and kosher salt in a bowl.
Whisk together eggs, maple syrup, coconut oil, and vanilla extract in a bowl.
Mix the wet and dry ingredients together until throughly combined with no chunks of almond flour remaining.
Stir the sliced apples into the batter and gently fold them into the batter.
Transfer the muffin batter into a muffin tin and decorate with apple slices and sprinkle with walnuts.
Bake in 325 degrees F oven for 35 minutes and let it cool on a wire rack before serving.