-
1
Heat the oven to 200 °C. Open the mussels by prying the shells apart by hand or by placing them briefly over high heat. Collect every drop of their liquor into a jug and set aside. Remove each mussel from its shell; reserve both meat and liquor. The liquor is the sole cooking liquid for the rice — taste it: it should be briny and clean.
-
2
Oil the base of a terracotta tiella or heavy oven dish generously. Lay the potato slices in a single, slightly overlapping layer — slices must be no more than 3 mm thick to cook through in the allotted time. Drizzle with olive oil.
-
3
Scatter half the mussels over the potato layer. Spread the raw, unwashed rice evenly over the mussels — the surface starch of the grain must remain intact to thicken the liquid as it cooks. Pour the reserved mussel liquor over the rice; if the mussels were small and the liquor sparse, add a splash of white wine or water so the rice is just covered. This is where the dish lives or dies: too little liquid and the rice will not cook through; too much and the top will never crisp.
-
4
Layer the sliced tomato and onion over the rice. Add the remaining mussels. Finish with a second layer of sliced tomato, the chopped parsley, and all of the grated Pecorino. Pour olive oil over the surface with an unsparing hand — the top layer must be well-anointed to caramelise and not dry out.
-
5
Place the dish, uncovered, on the middle shelf of the oven. Bake at 200 °C for 40–45 minutes. Do not cover at any point — the top must be exposed to direct oven heat to crisp. The dish is ready when the surface is deep golden and the Pecorino has coloured; confirm by inserting a thin skewer through all layers to the base — it should meet no resistance from the potato.
-
6
Remove from the oven and rest, uncovered, for 10–15 minutes. The terracotta retains heat and will continue to cook the interior during this time; account for this and remove the dish while a fine shimmer of moisture remains visible at the edges. Serve directly from the vessel.