Known in Italy as 'Riso, patate e cozze' this baked mussels with potatoes and rice dish is iconic in Puglia, Southern Italy. It's a baked dish of mussels, rice, sliced potatoes and tomatoes. The ingredients are added in layers and flavored with garlic, oil and parsley. See note below which will tell you how to prepare the mussels.
Open mussels with a knife, reserving any liquid that comes out as it is salty and flavourful and so important for this dish. If you prefer, you can cook the mussels in a saucepan for a few minutes until they just open and reserve all the cooking water. Keep mussels in one half of the shell for cooking, discarding the other half of the shell.