01 - Preheat your oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners or lightly grease each cup.
02 - In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and granulated sugar until well combined.
03 - In a blender, combine the fresh baby spinach, plain Greek yogurt, eggs, milk, vegetable oil (or melted coconut oil), vanilla extract, and optional lemon zest. Blend until the mixture is completely smooth and vibrant green.
04 - Pour the blended spinach mixture into the bowl containing the dry ingredients. Gently fold the batter together using a spatula until just combined. Be careful not to overmix.
05 - Divide the muffin batter evenly among the 12 prepared muffin cups, filling each cup approximately three-quarters full.
06 - Bake in the preheated oven for 18 to 20 minutes, or until a wooden toothpick inserted into the center of a muffin comes out clean.
07 - Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.