01 - Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease each cup.
02 - In a large mixing bowl, combine the all-purpose flour, baking powder, baking soda, salt, and granulated sugar. Whisk thoroughly to ensure all dry ingredients are evenly distributed.
03 - Transfer the fresh baby spinach, plain Greek yogurt, large eggs, milk, vegetable oil or melted coconut oil, and vanilla extract into a blender. If desired, add the zest of 1 lemon for enhanced brightness. Blend the mixture until it is completely smooth and vibrant green.
04 - Pour the liquid spinach mixture into the bowl containing the dry ingredients. Using a spatula, gently fold the batter until just combined, taking care not to overmix.
05 - Evenly distribute the muffin batter among the 12 prepared muffin cups, filling each cup approximately three-quarters full.
06 - Bake for 18 to 20 minutes, or until a clean wooden toothpick inserted into the center of a muffin comes out free of wet batter.
07 - Allow the muffins to cool in the muffin tin for 5 minutes before carefully transferring them to a wire rack to cool completely.