Preheat the oven to 350C (175C). Line the bottom and walls of two 8-inch (20cm) cake pans or spring forms with parchment paper.
In a large mixing bowl, mix together the almond flour, baking powder, ground nutmeg, ground cinnamon, xanthan gum, and salt. Set aside.
In another bowl, add sweetener and eggs and beat on high with a hand mixer for around 10 minute or until the mixture is pale yellow and tripled in size. Then, add the butter, heavy cream, and lemon juice. Beat on low for a few seconds or just until incorporated.
Add the dry ingredients to the wet ones. Using a spatula, fold in until no dry ingredients left.
Add the carrots, walnuts, and lemon zest and fold in again.
Divide the batter between two cake pans, spread evenly, and bake for around 30-35 minutes or until golden brown and a toothpick inserted into the center comes out clean.
When done, remove from the oven and leave to cool down in the cake pans for 5 minutes. Then, remove from the pans and place onto a rack and allow to cool down completely. If you plan to assemble the cake the next day, wrap each cake layer (completely cooled) with plastic wrap and refrigerate.