Cut the bananas in half widthwise and in half lengthwise.
Lightly coat the grill with cooking spray and preheat for 5 minutes.
Combine the sugar, cinnamon and walnuts in a small bowl.
Place the bananas in the grill width wise (horizontally) and spoon the cinnamon-walnuts over each slice.
Grill for 3-4 minutes, or until the bananas are warm and the cinnamon-walnuts are slightly glazed.