It's exactly the same as I have answered in this question: http://www.italki.com/question/307879 and here, "一" can be extended as the meaning "只要", too. In China, When/Once you talk about matters which happened a long time ago, you'll probably mention the fox.

一说起 once you mention (talk about)… once you talk about winter, you would probably think about the snow. 一說起冬天,就有可能想到雪 once you talk about the super hero films, you would probably mention the iron man. 一說起超級英雄電影,就可能談到鋼鐵人

When you are talking about Italy you will think of Pizza. When you are talking about=一说起