While cleaning the mussels, soak the rice in cold water with a pinch of salt.

Clean the mussels by removing all residues from the shell, then open them (without killing) and shell only halfway, carefully lifting the mussel from its natural adhesion to the shell with a small knife. Keep the liquid that drips from the mussels as they open, filter it, and set it aside.

Peel the potatoes and slice them into not too thin rounds. Place them in a bowl with cold salted water. Chop the cherry tomatoes into very small pieces.

Prepare a large bowl mixing breadcrumbs, grated cheese, minced garlic, chopped parsley, and two pinches of salt. Drain the rice.

Choose a baking dish (preferably aluminum) with fairly high sides, and line the bottom with sliced onion and a drizzle of oil. This is the start of assembling the dish. – Begin with a layer of potatoes, then mussels, then rice (making sure it's not too much and fits around and inside the mussels).

Sprinkle generously with the mixture from the bowl (cheese, breadcrumbs, etc.) and scatter some small pieces of tomato here and there.

Repeat with potatoes.

Finish with a layer of potatoes, add the mussel liquid, sprinkle with the mixture from the bowl. Finally, season generously with good olive oil, and pour water from the edges of the dish (pan) until almost covering all layers. The water should almost reach the top layer of potatoes but not quite.

Bake in a hot oven for 40/45 minutes. Check occasionally to make sure the water is enough for the rice to cook.