Line or spray 12 muffin tins and preheat your oven to 425℉.
In a medium mixing bowl, add the flour, flaxseed, baking powder, baking soda, ground cinnamon, ground nutmeg and salt. Whisk until combined and set aside.
In a large mixing bowl, add the eggs and maple syrup. Whisk until well combined. Then add the oil, yogurt, oat milk and vanilla extract. Whisk until combined.
Add the dry mixture to the wet mixture and gently fold until almost combined. If not using lemon juice and zest, add the blueberries to the batter. (If you are using, in the same medium mixing bowl, add your blueberries and zest your lemon and add the lemon juice. Stir together and then dump the blueberries into the batter.) Fold together until just combined.
Fill each muffin tin a little overflowing and then, if using, sprinkle turbinado sugar on top. Bake at 425℉ for 5 minutes and then lower the temperature to 350℉ and bake for an additional 10-12 minutes or until the muffins spring back slightly when touched. Allow to cool in the pan for 5 minutes before transferring to a wire rack to fully cool. Store in an airtight container for up to 3 days or in the freezer for up to 3 months and enjoy!