Preheat oven to 375 F. Line a cupcake tin with muffin liners (recipe makes about 12-14 muffins). Set aside.
For the topping, in a medium bowl combine the flour, walnuts, brown sugar, and salt until combined. Add the butter cubes and using your fingers, run the butter through the mixture until it resembles coarse crumbs. Set aside.
In a large bowl, combine the flours, baking soda, baking powder, and salt.
In another bowl, combine the mashed banana, sugar, egg, and melted butter. Add these ingredients to the bowl of dry ingredients and mix until just combined, with some flour streaks remaining. Add the walnuts and freeze dried strawberries and fold into the batter until well dispersed - try not to overmix.
Scoop the batter into the muffin liners and top with the crumb topping and additional freeze dried strawberry pieces. Place in the oven to bake for 18-22 minutes, until a toothpick placed in the center of the muffin comes out clean. Remove from the oven and remove from the tin after 10 minutes. Let the muffins cool completely on a cooling rack, then enjoy for breakfast, after a run, or for a not-too-sweet dessert.