Preheat the oven to 400°F (200°C). Line a large 6-cup muffin pan or a regular 12-cup cupcake pan with muffin liners or parchment paper. You could also use a silicone muffin pan.
Place the oats in a food processor or blender and blend until a coarse flour is formed. It should not be fine like regular flour but should have some texture. To a large mixing bowl, add the blended oats, baking powder, cinnamon and salt. Mix and set aside.
Roughly chop the apples, add to the food processor and pulse-blend until the apples are finely minced. If you don’t have a food processor, leave the apples whole and use a cheese grater to shred them. You can also substitute unsweetened apple sauce.
In a separate mixing bowl, add the peeled bananas and mash them with a fork or potato masher; leave a few chunks. Then, add the apples, maple syrup, lemon juice, vanilla and water. Stir to combine.
Add the wet mixture to the dry mixture; make sure they are well combined. Fill each section of the muffin tin all the way to the top. Because these muffins are so dense and don’t contain flour, they don’t rise much. So, you don’t have to worry about them overflowing.
Bake for 30 to 35 minutes until the tops are golden brown. Use a toothpick as a tester by inserting it into the center of a muffin. When it comes out clean the muffins are ready. Let sit for 5 minutes, then transfer the muffins to a wire rack for 5 minutes. Leftover muffins can be kept in the refrigerator for up to a week and enjoyed cold or warm.