Place clean mussels in a sauté pan with minced garlic, white wine and a pinch of salt on medium heat. Cover with the lid and cook for 3 minutes, until mussels begin to open.
Remove the top shell of the mussel, leaving the valve intact with the bottom shell. Keep all the juices from the mussels and place it aside.
Drizzle olive oil into the bottom of a Casserole and spread evenly.
Wash, peel and slice the potatoes. Add a layer of potatoes into the casserole. Add half of the sliced onions, chopped tomatoes, chopped parsley and parmesan cheese. Season with oregano, a pinch of salt and pepper.
Place the mussels evenly on top of the potatoes.
Add the rice evenly over the mussels. Pour the juices from the mussels and add water to slightly cover the rice layer.
Place another layer of sliced potatoes. Add the remaining of the sliced onions, chopped tomatoes, chopped parsley and parmesan cheese. Drizzle with olive oil and season with salt and pepper.
Bake in a pre-heated oven at 400°F for about 1 hour or until the potatoes are golden on top and water has been fully absorbed by the rice and potatoes.
Leave to rest for 10-15 minutes before serving.
Note: For every cup of rice, add two and a half cups of water. Add water to the edge of the casserole if needed, not to spoil the composition.