Preheat the oven to 400 degrees F.

In a medium saucepan combine the white wine and cleaned mussels. Place a lid on the pan and place over high heat. Cook, shaking the pan often for about 2 to 3 minutes or until the mussels are just opened. Remove the mussels from the pan and discard any unopened mussels. Remove and discard the top shell of each mussel leaving the meat in the bottom half of the shell. Strain the liquid through a fine mesh strainer and reserve. You should have about 3/4 cup.

Drizzle an 8 inch square baking dish with the oil. Layer the sliced onion on the bottom of the pan followed by a layer of potatoes overlapping them slightly to cover the onions. Press down firmly to flatten. Season the potatoes with 1/4 teaspoon salt, chopped garlic and the parmesan cheese. Sprinkle 1/4 cup of the rice over the cheese. Place the mussels on the half shell on the rice. It might be snug but nestle them in there. Sprinkle the remaining 1/2 cup rice over the mussels. Using your hands, crush the drained tomatoes over the mussels. Pour the reserved mussel liquid around and over mussels followed by the reserved tomato juices. Shingle the remaining potatoes over the top and season with the remaining 1/4 teaspoon salt. Pour the water around the edges. The liquid should come up just under the top layer of potato.

In a small bowl mix together the panko, olive oil, parmesan and oregano. Sprinkle the topping evenly over the casserole and cover tightly with foil. Bake for 45 minutes. Uncover the dish and broil on high to brown the topping, about 5 minutes. Allow the dish to rest for 10 minutes before serving.