Preheat the oven to 350°F (175°C). Grease and line an 8-inch round cake pan with parchment paper.
In a medium bowl, toss the sliced apples with lemon juice to prevent browning. Set aside.
In a large bowl, whisk together almond flour, coconut flour, powdered erythritol, baking powder, ground cinnamon, and salt.
In a separate bowl, beat the egg yolks with melted butter and vanilla extract until smooth and combined.
Add the wet ingredients to the dry ingredients and mix until a thick batter forms.
In a clean bowl, beat the egg whites with cream of tartar using an electric mixer until stiff peaks form.
Gently fold the beaten egg whites into the batter in three additions, being careful not to deflate the mixture.
Pour half of the batter into the prepared cake pan and spread evenly.
Arrange half of the apple slices over the batter in a single layer.
Pour the remaining batter over the apples and spread gently to cover.
Arrange the remaining apple slices on top of the batter.
In a small bowl, mix chopped pecans, granulated erythritol, and cinnamon to make the streusel topping.
Sprinkle the streusel evenly over the top of the apples.
Bake for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown.
Remove the cake from the oven and allow it to cool in the pan for 15 minutes.
Run a knife around the edges and transfer the cake to a wire rack to cool completely before slicing.