Preheat the oven to 350ºF and grease a mini muffin tin.
Into a blender container, add the banana, water, almond butter, egg, honey, spinach, rolled oats, baking soda, cinnamon, vanilla, and salt, in that order. (Liquids on the bottom help facilitate blending better.) Blend until very smooth, stopping to scrape the sides as needed.
Pour the green batter into the prepared mini muffin cups. I usually get 22 to 24 muffins out of this batch, depending on the size of the banana. If you like to measure, I use about 1.5 - 2 tablespoons of batter per muffin cup.
Put the muffin tin in the preheated oven and bake until the muffins rise and feel firm to a light touch in the center, about 15 to 17 minutes. The edges should turn lightly golden, and might crack a bit.
Remove the muffins from the oven and allow them to cool at least 30 minutes before removing them from the muffin tin. I find that using my fingers to twist them out of the tin helps them release easily, but you can also run a knife around the sides of the muffin to help get them out.
Muffins can be stored at room temperature for 24 hours, but I recommend storing them in an airtight container in the fridge for up to a week.