Preheat oven to 375F and line a 9x13" baking pan with parchment paper.
Cream butter, brown sugar and vanilla. Add egg, sourdough discard, and bananas or carrots and mix well.
Add oats, flour, baking powder, salt, cinnamon, and nutmeg. Stir in dried fruit, nuts or chocolate chips if you are adding them. Batter will be thick and sticky.
Spread batter evenly in the prepared pan.
Bake for 25-28 minutes at 375F. Lift by the parchment paper to remove from the pan and transfer to a rack to cool.