Preheat oven to 350º.
Drizzle 4 tablespoons of olive oil into a cast iron skillet.
Arrange a layer of onions in the bottom of the skillet and drizzle with olive oil.
Add a layer of potatoes seasoned with garlic, tomatoes, parsley, cheese, salt, and pepper. Drizzle with oil.
Add a layer of mussels seasoned with garlic and parsley. Drizzle with oil.
Layer the rice over the mussels, making sure to cover each mussel. Season with garlic, tomatoes, parsley, cheese, salt, and pepper. Drizzle with oil.
Add a final layer of potatoes.
Add the reserved liquid from when you opened the mussels. Add water so that the final layer of potatoes is just covered.
Sprinkle the breadcrumbs on top.
Place in the oven and bake at 350°F for 1 hour and 30 minutes.