Preheat oven to 375 degrees F. Place 12 muffin-cup liners in muffin cups and lightly coat with non-stick spray.Combine streusel ingredients: oats, flour, brown sugar, cinnamon, flaxseed and coconut oil. Toss with a fork.In a large bowl, combine dry ingredients for muffins: flour, baking powder, baking soda, salt and cinnamon. Stir well with a whisk.In a separate bowl, combine wet ingredients: egg, Greek yogurt, Truvia (or sugar), coconut oil, vanilla extract and lemon zest. Stir well with a whisk.Add Greek yogurt mixture to flour mixture and stir until just combined. Fold in blueberries.Divide batter evenly among prepared muffin cups. Sprinkle evenly with streusel topping. Bake for 22-24 minutes, or until a wooden pick inserted in the center comes out clean.Cool on wire rack for 15 minutes before removing from pan. Remove from pan; cool completely on wire rack.