Heat oven to 350 degrees. Place four baking cups in a muffin tin.
In a bowl, whisk together flour, baking powder, cinnamon, and salt. In another bowl, whisk together eggs, banana, and honey. Mix the wet ingredients into the dry, then fold in apple and walnuts.
Fill the batter to the tops of the lined cups and fill remaining cups halfway with water.
Bake 30 minutes, or until a tester comes out clean. Let cool on a wire rack.
Store muffins in an airtight container up to 2 days.