Preheat the oven to 350 degrees F. Spray a muffin tin with cooking spray.
Combine all dry ingredients (flour, oats, baking powder, baking soda, cinnamon, salt) in a large mixing bowl.
In a small bowl stir the wet ingredients (banana, apple, egg, milk, honey, and oil) together until the egg is well incorporated.
Pour the wet ingredients into the dry and stir gently until just combined. Avoid over- stirring as this can make the muffins tough.
With a large spoon, fill each muffin cup to the top (about 1/4 cup).
Add the streusel if using (see instructions below) with a small spoon, sprinkle approx 1/2 tsp of streusel on top of each muffin.
Bake for 14-16 minutes or until a toothpick or knife comes out clean. Let sit in muffin tin for 2-3 minutes then remove and let cool on cooling rack.