Preheat oven to 350 degrees. Grease a 9x13 inch baking pan (I used coconut oil).
In a medium sized bowl, combine all the dry ingredients (oats/muesli, spices, baking powder, salt).
In a separate bowl, combine the wet ingredients (banana, milk, vanilla, sweetener), starting with the mashed banana. Pour the wet ingredients into the dry, stir to combine.
Toss diced apples with lemon juice and another sprinkle of cinnamon, then fold these into the oat mixture.
Pour the oat mixture into the greased pan and sprinkle with chopped nuts. Bake 20-25 minutes.