Using a food processor, grind the granola coarsely to a crumb-like consistency. Pour into a mixing bowl and set aside.
Next, put the peeled bananas, cinnamon, and water into the food processor and blend into a slurry. Pour this mixture into the bowl with the granola and mix well using a spatula. The cereal soaks up moisture so add water as needed to keep the mixture moist and pliable.
Scoop out rounded portions of the mixture using a teaspoon, and evenly place them on a silicone sheet or pre-cut parchment paper for your dehydrator. Using the back of the spoon or the spatula, flatten out each portion into the shape of a cookie approximately ¼" thick.
Dehydrate the cookies at 135° F for approximately 8-10 hours. For chewier cookies, dry for less time. For crisper cookies, dry longer.
About halfway through the dehydrating process flip the cookies over and remove the silicone sheet or parchment paper. Continue to dehydrate on the mesh tray until done.
When finished, store the cookies in a zip lock bag in the freezer or the pantry. Each batch makes about a dozen cookies.