If you don't have a powerful blender, start by placing the milk and spinach in a blender. Blend until the greens are totally pulverized, for the best texture. (You can skip this step if using a high-speed blender.)
Add in the rest of the ingredients, and blend until very smooth. You may need to add up to a 1/4 cup more milk or water to help everything blend smoothly, depending on the size of the banana you use. The riper the banana you use, the sweeter this smoothie will be. If you need to add sweetness, you can always add a pitted Medjool date, or a splash of maple syrup, or honey (if not vegan), and blend again. Serve right away for the best taste and texture.
Leftover smoothie can be stored in an airtight container in the fridge for up to 48 hours, but the taste and texture will change over time. Be sure to check out the freezer bag tips in this post, for another make-ahead option.
Nutrition information is for the whole batch. This information is automatically calculated, and is just an estimate, not a guarantee. To make a more filling smoothie, feel free to add another tablespoon of nut butter, or extra frozen fruit + more liquid to blend.I typically use fresh baby spinach in this recipe, but you can use frozen spinach if you prefer. In that case, start with just a 1/4 cup of frozen spinach (since it is more compact) and then you can add more, to taste, as you blend. See more tips in the full post.