Preheat oven to 375 degrees F (190 C) and lightly grease a standard size muffin tin (will make 10-11 muffins as original recipe is written // adjust if altering batch size)
Prepare flax egg in a large mixing bowl. Let set for 5 minutes.
To the flax egg, add banana and mash, leaving just a bit of texture.
Add brown sugar, baking soda, salt and whisk for one minute.
Stir in vanilla, melted butter and mix.
Add flour and oats and stir with a spoon or spatula until just combined. Lastly, fold in walnuts (optional).
Divide batter evenly among 10-11 muffin tins (adjust if altering batch size), filling a generous 3/4 full (I fill mine practically full for aesthetic effect).
Quickly wipe your mixing bowl and add crumble ingredients. Prepare crumble top by mashing ingredients together with a fork until crumbly like wet sand. Generously top muffins with streusel. You may have leftovers (reserve for future muffins or add to crumbles, pies, quick breads, etc.).
Bake for 17-22 minutes or until tops are golden brown and a toothpick or knife comes out clean. Let cool for a few minutes, remove from muffin tin and serve warm. Once completely cooled, store in a covered container at room temp for several days. Freeze for longer term storage.