Preheat the oven to 375F. Line a baking sheet with parchment paper; set aside.
In a small bowl, stir the flax with 2 tablespoons of filtered water (this is known as a “flax egg”). Refrigerate for 5-10 minutes while you prepare the rest of the ingredients - the resulting "egg" should be thick and slightly gelatinous.
Pour 1 cup of the oats into a food processor and grind them to a fine meal, 1-2 minutes.
Transfer the ground oats to a medium bowl, along with the remaining ¾ cup oats, baking powder and salt. Whisk to combine.
In a separate bowl, stir together the banana, coconut oil, sugar, matcha and the flax egg. Fold the oat mixture into the banana mixture, stirring just until combined. Fold in the pistachios.
Spoon the mixture onto the prepared baking to make 8 cookies, spacing them 1-2 inches apart. Bake until lightly golden, 12-14 minutes. Transfer the cookies to a cooling rack and let sit for 10 minutes before eating.
If not eating immediately, place cooled cookies in an airtight container and store in a cool dry place for up to 3 days.