Scrub and clean the mussels, removing beards. Open them by hand or briefly over heat, keeping the half shell with the meat and reserving all their juices.

Slice the potatoes and onions thinly so they cook evenly. Cut the tomatoes in ⅛ and season them lightly with salt, olive oil, and pepper.

In separate bowls, season the potatoes with salt and a drizzle of olive oil. Mix the tomatoes and onions together with a touch of salt and pepper.

In a small bowl, combine the minced garlic, chopped parsley, Pecorino, and Parmigiano: this will be your flavor layer between each tier.

Oil a round or oval baking dish generously. Start with a layer of the tomato and onion mix at the bottom so it keeps the potatoes from sticking.

Add a layer of potatoes, slightly overlapping like roof tiles. Spoon over a little of the tomato mix again and sprinkle with some of the cheese and herb blend.

Now arrange the mussels, still in their half shells, across the surface. Dust them with another layer of the cheese mix, then scatter the raw rice evenly on top.

Add another layer of tomatoes and cheese, followed by the remaining potatoes. Finish with a last layer of tomatoes and a final sprinkle of the cheese mixture.

Drizzle everything with olive oil before adding the liquid. Combine the mussel juice with just enough water to reach the top layer without submerging it, the dish should steam and not boil.

Pour the liquid in gently along the edge of the dish so the layers stay intact. Finish with another drizzle of olive oil.

Place the tiella in a preheated oven at 220°C (450°F) for 10 minutes to give it color. Lower the temperature to 180°C (365°F) and bake another 40 minutes, until the top is golden, the rice tender, and the edges sizzling.

If it browns too quickly, cover loosely with foil. Let the tiella rest for 10–15 minutes before serving.