Preheat the grill at its highest settings. Arrange the bananas in a buttered baking tin. Gently heat the butter and lemon juice in a saucepan, pour over the bananas, sprinkle with the sugar.
Grill until the sugar has caramelized, that is for about 4-5 minutes. When it is done remove and sprinkle walnuts. Serve immediately with vanilla ice-cream.