"Cookie" might be a loose term for these, but if you are looking for a very healthy cookie, and you don't mind if they are more like a healthy cracker, you should make these--they are surprisingly good.
2 ripe bananas
1 cup oats
Optional (but kind of recommended) Add one or more of the following:
sea salt (to sprinkle over)
cinnamon (to sprinkle over)
raisins or other dried fruit
Mash bananas and mix with oats. Mix in any add-ins that you choose. Spoon onto cookie sheet and bake for 15 minutes or more at 350 (or more). I liked them best when cooked at a higher temperature (like 400) for 20 minutes, flattening them with a spatula part way through, and flipping them over. They were a little crispy this way. This is obviously a very flexible recipe.
These ones had craisins and vanilla in them, and before baking, I sprinkled them with cinnamon and sea salt.
This is how they look after baking--the ones on the left I cooked longer at a higher temperature, and the ones on the right I baked for 350 for 15 minutes.