Preheat oven to 350F. Spray muffin tin with non-stick spray or use a silicone muffin pan. (I find it best not to use paper liners–the muffins will stick to them.)
Mix together dry ingredients (flour through flax seeds) in a large bowl. In a smaller bowl, mix wet ingredients (yogurt through agave). In an even smaller bowl, mix the sugar and cinnamon.
Pour the wet mixture into the flour mixture, add the chopped apple if you're using it, and stir just until thoroughly moistened. Do not beat or overmix. Spoon about 1 heaping tablespoon of batter into each muffin cup. Sprinkle each half-full muffin with 1/2 teaspoon of cinnamon sugar and fill with remaining batter. Sprinkle remaining cinnamon sugar over each muffin. Take a toothpick or thin knife, insert it into the batter and swirl gently.
Bake for 17-22 minutes, until a toothpick inserted into the center of a muffin comes out clean. Cool for a few minutes before serving.
Notes
A reader suggested adding chopped up apple to these, and I have to admit it adds a lot to the recipe. But nutritional info does not include the apple.