Line a baking sheet with parchment. Arrange banana slices and prepared berries in a single layer. Freeze until very cold but not frozen solid, about 10 minutes.
Blend all ingredients till smooth. Taste for sugar; add if needed. Serve immediately.
Note: If your berries are already frozen, use as is and don’t freeze the banana.
Keywords: Bananas, Berries, Beverages, Breakfast or Brunch Dish, Cook's Illustrated, Fruit, juice, Lemon, milk, Smoothie, Yogurt