Halve your cherry tomatoes, Peel and thinly slice into rounds your potatoes. Your parsley should be fresh I always use italian parsley chopped. You will need salt and coarsely ground black pepper to taste.
Preheat oven to 375.
Scrub and rinse the mussels well. Pull awa the beards and discard them. Throw away any mussels with cracked shells. Put mussels in saute pan, add wine, cover the pot ad steam them until the shells open. Drain in a cheesecloth lined strainer or colander over a bowl. RESERVE THE LIQUID.
Remove the top shell of each mussel. Set the mussels aside. Spreadthe olive oil in a casserole dish. Arrange a layer of onions over the olive oil. Add a layer of tomatoes and layer of potatoes. Place the mussels in their shells over the potatoes in a single layer.
Salt and pepper the mussels to taste.
Combine the cheese, bread crumbs, and parsley together in a small bowl. Sprinkle some of the mixture over the mussels. Continue making layers of onion, tomatoes, potatoes, mussels and cheese.
Pour reserved mussel liquid along the side of the pan. Pour the beaten egg over the top of the casserole.
Bake uncovered until the potatoes are browned, about 45 minutes. Use a spoon to scoop and serve.