Preheat oven to 375ºF. Lightly spray a 12-cup standard muffin tin with nonstick spray.
Heat olive oil in a large skillet over medium heat. Add Italian sausage and cook until browned, about 5 minutes, breaking up the sausage as you go. Drain off excess fat and add marinara sauce to the sausage. Season with salt and pepper to taste.
Bring a large pot of salted water to boil over high heat. Blanche the ravioli for 1-2 minutes, just until they are slightly more pliable. They will continue to cook in the oven. Drain the ravioli and dab off any excess moisture.
Place one large ravioli into each of the 12 muffin tins, pressing carefully to make sure that it fills the whole bottom. Fill each cup with 1 tablespoon marinara mixture and 1 tablespoon mozzarella cheese; repeat with one more layer, ending with mozzarella on top.
Place into oven and bake for 10-12 minutes, or until the cheese has melted and the ravioli are golden brown.
Serve immediately, garnished with chopped parsley, if desired.