Preheat oven to 375 degrees. Grease a muffin pan and set aside.
Whisk together first 7 ingredients (milk through salt) in a medium bowl. Stir in oats, walnuts and bananas. Divide evenly among muffin tins. Sprinkle brown sugar evenly over muffin tins.
Bake for 20 minutes or until centers are set. Cool for at least 10 minutes before removing from pan.
Add stir ins like extra milk and almond butter, if desired.
Notes
Allow to cool completely before freezing. Recipe adapted from Serious Eats.