Preheat oven to 375 degrees F. Grease or line a muffin tin.
Combine all ingredients in a large, high-powered blender in the order they are listed: almond milk, maple syrup, eggs, Greek yogurt, vanilla, bananas, spinach, old fashioned oats, baking powder and soda, and sea salt. Blend until smooth, about 1-2 minutes. The batter should look like a green smoothie.
Pour batter to the top of each muffin cup. Put the muffin tin in the oven and back for 20-22 minutes, or until the muffins are brown around the outside and a toothpick that you put in the center comes out clean.
Transfer the muffins to a wire rack to cool. Enjoy warm or chilled. Store in a sealed container on your countertop or in the fridge for 5-7 days. You can also freeze these muffins and thaw them completely before eating.
You can try to make these muffins as mini-muffins. Just check to see if they are done around 15-17 minutes, instead of at 20 minutes.