Mussel, Potato, and Rice Gratin

Directions

• Rinse the mussels and scrub them under cold running water. Remove the beard from each one by grabbing it with your fingers and pulling it toward the hinge of the shell, then discard.
• Discard any open mussels. Using an oyster shucking knife or a dull thin, short knife, open the mussels: Working over a bowl, hold each mussel firmly, using a towel to protect your hand, and slide a knife in between the two halves of the shell. Run the knife around the inside of the shell to cut through the hinge muscle. Once you hear a clack, the mussel can be easily opened; let the mussel liquor drain into the bowl. Remove and discard the empty top shell and leave the mussel in the bottom shell. As you go, arrange the open mussels, still in their bottom shells, on a rimmed baking sheet.
• Drizzle ¼ cup/60 ml of the olive oil over the bottom of a 12-inch/30 cm round casserole. Scatter half of the onions over the bottom of the pan, then top with half of the potatoes. Distribute half of the cherry tomatoes on top and sprinkle them with half of the parsley and garlic. Sprinkle one-third of the grated Pecorino Romano over, season with salt and pepper, and drizzle with another ¼ cup/60 ml oil.
• Arrange the mussels, in their shells, on top of the cherry tomatoes in an even layer; reserve a dozen mussels for the final layer. Scatter the rice over the mussels, making sure to fill their shells. Sprinkle with half of the remaining pecorino, season with salt and pepper, and drizzle with another ¼ cup/60 ml oil.
• Top the rice with the remaining potatoes, then arrange the remaining cherry tomatoes and the reserved mussels on top. Scatter the onions ove the tomatoes and sprinkle with the remaining parsley and garlic. Finish with the remaining pecorino and drizzle with the remaining ¼ cup/60 ml oil.
• Preheat the oven to 400°F/200°C.
• Strain the mussel liquor through a fine-mesh sieve and pour it into the pan. Then add as much salted water as needed to just reach the top layer of potatoes. Use this proportion for the salted water: 1 teaspoon/5 g fine sea salt per 4 cups/1 L water. Do not add too much water, or it will create a watery gratin. There is enough if when you gently press the last layer of onions, you see liquid surfacing.
• Place the pan over medium heat and bring the liquid to a boil. Remove from the heat, cover the pan with aluminum foil, transfer to the oven, and bake for 40 minutes. Remove the foil and bake for 30 more minutes, or until all the liquid has been absorbed and the potatoes are golden brown.
• Remove from the oven and let stand for at least 2 hours before serving, so that all the flavors can mingle. Reheat gently in a low oven before serving, or enjoy at room temperature.
• Any leftovers can be left in the fridge for a day.