1) Preheat the oven to 375°F. Grease a 12-cup muffin pan, or line the pan with muffin papers, and grease the papers.
2) Whisk together the the flours, flax, sugar, salt, cinnamon, baking powder, and xanthan gum in a large bowl.
3) Whisk together the melted butter or oil, eggs, and buttermilk.
4) Add the buttermilk/egg mixture to the dry ingredients, whisking until everything is well combined. A few lumps are fine.
5) Gently stir in the chopped apples and nuts and/or raisins.
6) Scoop the batter by the generous ¼-cupful into the prepared muffin cups; a muffin scoop works perfectly here. Sprinkle the tops with coarse sugar, if desired. Set aside to rest for 10 minutes.
7) Bake the muffins for 24 to 28 minutes, until a center muffin feel set when pressed lightly in the middle and is lightly browned on the edges. The tops of the muffins won't change color much.
8) Remove from the oven, and let sit for 5 minutes to firm up slightly before tipping out of the pan onto a rack to cool.
Yield: 12 muffins.