Preheat oven to 400°F. Place rack in middle position. Divide the garlic, onion, tomatoes and parsley into four equal parts, separate from each other. Divide the potato slices into three equal parts.
In a large oven safe pot, spread one part each of the sliced onions, tomatoes, garlic and parsley over the bottom of the pot. Add a drizzle of olive oil. Fan half of the potato slices on top of the first layer until fully covered.
For the second layer, spread one part each of the onions, tomatoes, parsley, and garlic. Sprinkle a layer of cheese on top and season with black pepper. Add a small drizzle of olive oil and then fan the potato slices on top of the second layer.
For the third layer, spread one part of the onions, tomatoes, and parsley. Sprinkle a layer of cheese on top and season with black pepper.
Add a light drizzle of olive oil and then place down a layer of the mussels in the half shell.
Spread the Arborio rice evenly over top of the mussels.
For the fourth layer, add the remaining onion, garlic, tomato, and parsley. Sprinkle the cheese on top and season with black pepper. Add a small drizzle of olive oil and then fan the potato slices on top.
Slowly pour in the water until it reaches just below the last layer of potatoes. Sprinkle another layer of cheese on top. Cover the pot with a lid and then place in the oven for thirty minutes.
After thirty minutes, remove the lid from the pot and allow to cook for another thirty minutes uncovered.
Remove from the stove and enjoy!