Preheat the oven to 350°F (180°C). Grease a 8½- by 4½-inch loaf pan liberally with ½ tablespoon butter.
Make the cake: In a medium bowl, whisk together the almond and coconut flours, flax seed, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and the granulated sweetener until light and fluffy, about 2 minutes. Add the sour cream and the vanilla and beat until combined. Add the eggs one at a time, mixing after each.
Add the flour mixture in 3 additions, mixing each addition on low until incorporated and thoroughly combined. Gently fold in the diced strawberries. Pour the batter into the prepared loaf pan, spread evenly, and bake until light golden brown, firm to the touch, and no longer jiggly, 50 to 55 minutes.
Let cool in the pan for 20 minutes before gently turning out onto a wire rack to fully cool, about 1 hour.
Meanwhile, prepare the garnish: In the clean bowl of a stand mixer fitted with the balloon whisk attachment, combine the heavy cream, powdered erythritol, and vanilla. Whisk on high until soft peaks form. Slow the speed to medium and very briefly whip for a few more seconds until peaks firm up; do not over whip. Refrigerate until ready to serve.
To serve, slice the cake and garnish each slice with whipped cream and sliced berries.