For dressing, in a blender container combine sliced bananas, lemon yogurt, sugar, and lemon juice; cover and blend for 15 to 20 seconds or until smooth. Chill dressing, covered, for up to 2 hours. To serve, line salad plates with lettuce; arrange fruit on top. Top with dressing. Sprinkle with walnuts. Makes 8 side-dish servings.