In a large mixing bowl, beat together the sugar, vanilla and the eggs until frothy.

Add in the oil and beat until a silky texture is achieved.

Sift the flour, cinnamon, baking soda, salt, and xanthan gum into a medium mixing bowl and mix together.

Add the egg mixture and beat until combined.

Gently stir in the rhubarb or apples and nuts using a wooden spoon.

Pour batter into a 9x13 greased pan, then sprinkle the top with sugar and cinnamon.

Bake at 375°F for about 45 minutes or until cooked through.