Open all the mussels and leave the fruit on the half shell. Save any juice that comes out of the mussels.
Take a large bowl and add the wine and 5 Tbs of EV olive oil. Add the juice from the mussels.
Chop the garlic and onions and add to the bowl. Add the tomatoes and the chopped parsley.
Take a 12" x 31/2" round oven pan and cover the bottom with slices of onions and tomatoes from the bowl. Cover the onions with a layer of the potatoes.
Lay over the potatoes a layer of mussels.
Put 1/2 Tbs of rice on each mussel. Then spread some rice over the entire surface.
Repeat another layer of onions, potatoes, mussels, rice. On the very top cover with the remaining potatoes and vegetables.
Pour into the pan the liquids until all the content is covered by 1/2 inch.
If the liquids are not sufficient add some hot water.
Top with a generous sprinkle of Pecorino cheese.
Cover the pan with aluminum foil and cook for 50 minutes at 350F on the middle shelf of your oven, then remove the foil and cook for an additional 10 mins. on the top shelf of the oven.
Then put the pan under the broiler for a few minutes to give it the desired color.