Preheat oven to 350 degrees and lightly oil a 12-muffin baking pan. Whisk the ground flax with 3 tablespoons warm water and set aside. In a medium sized bowl, whisk together the milk and vinegar.
In a large mixing bowl, combine together the flour, oats, baking powder, salt, cinnamon, and chia seeds.
Add the mashed banana, sugar, and oil to the milk & vinegar mixture. Stir in the flax mixture. Add these wet ingredients to the dry ingredients. Mix with a spatula until the batter is evenly combined. Don't overmix: a few small lumps are OK!
Add 1/3 cup batter to each well in the muffin pan. Sprinkle the tops with extra rolled oats and bake for about 22-25 minutes, or until muffins are browning and a toothpick comes out mostly clean.