Preheat oven to 350 degrees F.
Place sweet potatoes on a baking sheet and roast until tender, about 1 hour. Remove from heat and let cool until cool enough to handle. (Keep oven at 350 degrees F). Peel and mash sweet potatoes in a bowl until smooth. Stir in the butter, stock, salt, pepper, and cinnamon. Transfer to a casserole dish. Cover the surface of the potatoes with the banana slices by laying them on top, slightly overlapping in concentric circles. Sprinkle the brown sugar evenly over the top. Bake for 20 minutes or until the top is browned.