Preheat your oven to 350°F. Grease a 9 inch round cake pan, and set it aside.
Prepare the strawberries
Line a large, rimmed baking sheet with parchment paper and set it aside.
In a medium-size bowl, place the prepared strawberries, 2 tablespoons granulated sugar, and pinch of salt. Toss to coat the berries.
Place the coated berries in an even layer on the baking sheet, cut side down.
Place the baking sheet in the center of the preheated oven and roast the strawberries for 35 minutes or until they are very soft and their juice is reduced to a thick liquid.
Remove the strawberries from the oven, and set aside to cool briefly.
Make the cake
Reduce the oven temperature to 325°F.
In a large bowl, place the flour blend, xanthan gum, salt, baking powder, baking soda, and granulated sugar, and whisk to combine well.
Add the butter, and press down on the mixture with the back of your mixing spoon until the butter is mixed throughout the dry ingredients. The mixture should resemble coarse sand.
Add the egg, vanilla, and milk or buttermilk, and mix to combine. The batter will be thick, especially if you used buttermilk.
Transfer the batter to the prepared round cake pan, and smooth into an even layer with a wet spatula.
Place the roasted strawberries, cut side down, on top of the cake batter next to one another in nesting circles, with a common center. Fit as many of the prepared cut berries as you can without overlapping.
Sprinkle the top of the cake with a few granules of coarse sugar, if using.
Place the cake in the center of the 325°F oven.
Bake for about 40 minutes, or until the cake is light golden brown, has begun to pull away from the sides of the pan, and it springs back when pressed gently in the center.
If the cake begins to brown too quickly, cover it loosely with aluminum foil. A cake tester won’t come out completely clean because of the strawberries, especially in the very center.
Remove the cake from the oven and allow it to cool for at least 10 minutes before transferring it to a wire rack to cool completely.