Make the strawberry swirl: In a small saucepan over medium heat, combine chopped strawberries, powdered sweetener, and lemon juice. Stir occasionally and cook until the strawberries break down and the mixture thickens, about 5-7 minutes. If the mixture is too juicy, add xanthan gum to thicken. Set aside to cool.

Prepare the loaf batter: Preheat the oven to 350°F (175°C). Grease a loaf pan with butter or line it with parchment paper. In a large bowl, whisk together almond flour, coconut flour, baking powder, and salt. In another bowl, combine melted butter, granulated keto sweetener, eggs, almond milk, and vanilla extract. Mix until smooth.

Combine the batter and strawberry swirl: Add the wet ingredients to the dry ingredients and stir until fully combined. Pour the batter into the prepared loaf pan and smooth the top. Spoon the strawberry mixture over the batter in dollops, then use a knife or skewer to swirl the strawberry mixture through the batter, creating a marbled effect.

Bake the loaf: Bake for 40-50 minutes, or until a toothpick inserted into the center comes out clean. If the top begins to brown too quickly, cover it with foil and continue baking until done.

Cool and serve: Let the loaf cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely. Slice and serve.