Pre-heat oven to 350 °F (175 °C). Lightly oil an (8") baking pan with cooking spray; set aside.
Peel, core, and grate apple. Peel and grate sweet potato.
In a large bowl, whisk together flour, baking soda, salt, cinnamon and nutmeg; set aside.
In a second large bowl, whisk together sweetener (like Truvia) and egg white until well combined. Stir in apple, sweet potato, and vanilla and then add apple mixture to flour mixture and stir to combine.
Transfer batter to prepared pan and bake until cake pulls away from the edges and is deep golden brown, 40 to 50 minutes.
Set aside to let cool and then cut into squares and serve.
There are 91 calories in 1 serving of Sweet Potato Apple Cake.