Put the banana pieces in a large skillet, pour the maple syrup over them, and tuck the rosemary sprigs in between. Bring the mixture to a low boil over medium heat and cook for about 1 minute. Turn the bananas over with two forks and cook for 30 more seconds. Scoop the ice cream into shallow dishes. Lift the bananas from the syrup with the forks and lay 3 pieces around each scoop of ice cream. Boil the syrup left in the pan for a minute or two until it thickens slightly, then swirl in the butter until it melts and incorporates. Use a fork to hold back the rosemary sprigs as you pour the sauce from the skillet over the bananas and ice cream. Sprinkle with the walnuts.