Preheat your oven to 350°F (175°C). Line a mini muffin tin with paper liners or lightly grease it.
Wash the spinach thoroughly. If using frozen spinach, thaw it and squeeze out excess moisture.
Place the spinach in a blender or food processor.
Add a small amount of water (or a bit of applesauce or yogurt) to help with blending. Blend until smooth.
In a large bowl, combine the applesauce, Greek yogurt or buttermilk, honey or maple syrup (if using), egg, and vanilla extract. Mix until well combined.
Stir the blended spinach into the wet ingredients until well incorporated.
Add in the the flour, oats, baking powder, cinnamon (if using), and a pinch of salt. Mix until just combined.
Spoon the batter into the mini muffin tin, filling each cup about 3/4 full.
Bake for 12-15 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the mini muffins to cool in the tin for a few minutes before transferring them to a wire rack to cool completely.
Serve and enjoy!