Pour the almond milk, lime juice, maple syrup, and salt into a 1.5 L / 6-cup blender jar. Scoop in the avocado flesh, then add the chopped cucumber, spinach, and mint. Top with the frozen banana slices and ice.
Blend on low for 15 seconds, then on high for 45 to 60 seconds, until the smoothie is completely smooth with no green flecks.
Taste it. Add 1 tsp more maple syrup if it tastes flat, or a squeeze of lime if it tastes too mellow, then blend for 5 seconds. If it's too thick to pour, blend in 2 tbsp of almond milk.
Pour into two tall glasses and drink right away, within 30 minutes, while it's thickest and brightest.