Wash the apple, remove the stem, core and seeds, and cut the flesh into small pieces. Peeling is optional for a high-speed blender.
Add milk, Greek yogurt and apple pieces to the blender. Blend 30 to 45 seconds until the peel is finely broken down.
Add frozen banana, oats, nut butter, cinnamon, vanilla, lemon juice and salt.
Blend on high for 45 to 60 seconds, stopping once to scrape the sides if needed.
For a thinner smoothie, add milk 1 tablespoon at a time. For a colder smoothie, add a few ice cubes and blend briefly.
Divide between two glasses and drink promptly, before the apple and oats continue thickening.