Preheat the oven to 350°F (175°C). Grease an 8-inch round cake pan and line the bottom with parchment paper.
In a medium bowl, toss the sliced apples with lemon juice and 1/2 teaspoon ground cinnamon. Set aside.
In a large bowl, whisk together almond flour, coconut flour, powdered erythritol, baking powder, salt, and remaining 1/2 teaspoon ground cinnamon and nutmeg.
In a separate bowl, beat the eggs, then add melted butter and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until fully combined into a thick batter.
Spread half of the batter evenly into the prepared cake pan.
Arrange half of the apple slices evenly over the batter layer.
Spread the remaining batter over the apples, then top with the remaining apple slices.
In a small bowl, combine chopped pecans and granulated erythritol to create the streusel topping.
Sprinkle the streusel evenly over the top of the cake.
Bake for 45 to 50 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Remove from the oven and let cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before serving.