Preheat oven to 400 degrees. Grease a 12” cup muffin pan. Sift flour, sugar, baking powder and salt into a medium bowl, make a well in centre. Mix milk, egg, oil, banana and walnuts in a small bowl. Pour mixture into well. Mix batter just until moistened; do not overmix. Spoon batter into prepared muffin cups. Bake until a toothpick inserted in centre comes out clean or until muffins are golden brown, about 15 minutes.