It's really in Montepertuso, but they have a shuttle service which will collect you from Positano, and drop you back. Or the frequent buses from Positano to Montepertuso and Nocelle will get you there. It has three areas - outside in the square, outside on the terrace, and inside. The latter two are fancy, and the former informal. We had lunch on the square and (different day) dinner on the Terrace. It's a beautiful and romantic place, and the food is absolutely exceptional. We're both vegetarian but had no problem finding food to choose. On our second visit my wife wanted the risotto I had previously, which was off the menu as the risotto changes daily, but they were very happy to make that particular risotto especially for her. As mentioned elsewhere it's family run, and the service couldn't be more friendly. What a wonderful restaurant.
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