Preheat oven to 350º.
Line two baking sheets (or cake pans) with parchment paper. Set aside.

Mix dry ingredients in a large mixing bowl. Set aside.

Put the vanilla, maple, and water into a saucepan. Whisk together over low heat.

Add the mashed banana to the saucepan, whisking into the rest of the goo.

Keep whisking wet ingredients over low heat until it begins to thicken into a chunky syrup. (About 2 minutes)

Remove saucepan from heat and pour contents over dry ingredients. Use a wooden spoon to mix, coating all of the dry pieces. This may look a little bit dryer than you expect it to.
It should form a ball when pressed together. If it doesn't, add a little bit of water. If it's too wet, add a little bit of oatmeal.

Using a wooden spoon or silicone spatula, press the granola into the bottom of the baking sheets. You want it to be a thin layer.
If you prefer less-chunky granola, spread it out. I like chunks, so I press it closely together.

Bake for about 20 minutes, total.
Begin checking it at about 15 minutes.
If you don't want big chunks, shake the pan to separate the pieces. 
When it's done, it should be mostly golden brown, with some darker brown chunks along the edges.

Remove from oven and place the baking sheets on a cooling rack. As it cools, it will become crunchier. Store in an air-tight container and hide from freeloading kids.