01 - Place Greek yogurt, milk, avocado, banana, spinach (if using), honey or maple syrup, lime or lemon juice, vanilla extract, and ice cubes in a blender.
02 - Blend on high speed until completely smooth and creamy, stopping to scrape down the sides as needed.
03 - Taste the smoothie and adjust sweetness or thickness by adding more honey for sweetness or additional milk for a thinner consistency, if desired.
04 - Pour into glasses and serve immediately while chilled. For meal prep, store in sealed jars in the refrigerator up to 24 hours and shake well before drinking.