Preheat the oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
In a medium bowl, toss the sliced apples with lemon juice, ground cinnamon, and 2 tablespoons of erythritol. Set aside to marinate.
In a large bowl, whisk together almond flour, coconut flour, baking powder, salt, and the remaining 2 tablespoons of erythritol.
In a separate bowl, beat the eggs until frothy. Add the melted butter, sour cream, and vanilla extract, mixing until combined.
Pour the wet ingredients into the dry ingredients and stir until a smooth batter forms.
Spread half of the batter evenly into the prepared pan.
Arrange half of the apple slices over the batter in an even layer.
Spread the remaining batter over the apples, smoothing the top with a spatula.
Arrange the remaining apple slices on top of the batter.
In a small bowl, combine the chopped pecans and cold cubed butter. Using your fingers or a pastry cutter, crumble the butter into the pecans until the mixture resembles coarse crumbs.
Sprinkle the pecan crumb topping evenly over the apples.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean and the topping is golden brown.
Remove from the oven and allow the cake to cool in the pan for 15 minutes before releasing the springform and transferring to a wire rack to cool completely.