Preheat oven to 375°F and grease an 8x8" square pan/ spray with nonstick spray. Set aside.
To a large bowl, add the overripe bananas. Mash the bananas until they're fully broken down (I usually just use a fork for this, it should be quick and easy if the bananas are very ripe!).
3 bananas
To the mashed bananas, add the rest of the wet ingredients - brown sugar (or brown sugar substitute), eggs, vanilla, and milk. Stir until evenly combined.
1/4 cup brown sugar, 2 large eggs, 1 teaspoon vanilla extract, 2 cups milk of choice
Add the dry ingredients (oats, flaxseed, baking powder, cinnamon, salt, and nutmeg) to the wet ingredients and stir until combined.