Preheat oven to 400F. Grease a muffin tin with cooking spray.Place flax and water in a small bowl. Stir and set aside.To make the streusel, place walnuts, granola, and coconut sugar in a small bowl, stirring to combine. Add coconut oil and mix together using your fingers until you have a crumbly mixture. Set aside.Place bananas and vanilla in a large mixing bowl and mash with a fork. Stir in flax mixture.Add almond meal, oats, granola, salt, cinnamon, and baking soda. Stir until just combined. Fold in blueberries.Spoon mixture evenly into muffin tin. Top each muffin with a sprinkling of the streusel mixture.Bake for about 25 minutes, or until a fork inserted into the center comes out clean. Allow muffins to cool for at least 20 minutes in tin on a cooling rack before removing from tin.Store in an air-tight container in the refrigerator.