Add the frozen bananas, protein powder, and spices to the blender and let it sit for a few minutes for the bananas to soften.
Zest the lemon and add it to a blender.
Remove the pith fron the lemon and add the rest of the lemon to the blender, removing seeds if you find them. Blend until smooth.
Add a cup of frozen crushed ice and ½ cup of water. Let it sit for a minute then blend on low speed to break up the ice. Gradually increase speed until the mixture becomes smooth. Take a taste test. If you'd like it to be sweeter, add a bit of stevia or maple syrup.
Pour the smoothie into glasses, top with lemon slices, and enjoy!
You can refreeze any leftover slushies. Remove from the freezer and allow it to sit at room temperature until it softens to a slushie consistency.
Notes
Optional additions
Add a scoop of protein powder to give the smoothie an extra boost of protein.
Throw in a handful of spinach for a nutrient boost and a slight green color.
A tablespoon of maple syrup can be added for a touch of sweetness.