3 cups chopped romaine lettuce
1/3 cup milk, or more as needed
4 frozen strawberries, or more to taste
1 frozen banana, cut into chunks
1/4 teaspoon vanilla extract, or to taste (optional)
Put romaine lettuce into the bottom of a blender pitcher; add enough milk to cover completely and blend on High until smooth.
Drop one strawberry at a time into the blender while still running on High and allow the berry to blend completely before adding the next. Blend one banana chunk at a time into the mixture in the same manner as the strawberries. Thin the smoothie with additional milk to keep smoothie blending properly. Blend vanilla extract into the smoothie.