Preheat oven to 350 degrees F.

Spread a 1/4 cup of sauce on the bottom of a large cast iron pan and set aside.

Bring a large pot of water with a pinch of salt to a boil. Cook jumbo shells for 2 minutes less than package instructions suggest. Drain pasta.

While pasta is cooking, mix together ricotta, romano, egg, spinach, salt, and pepper in a medium bowl. Fill pasta shells with filling, making sure they're still able to close. Place filled shells open side down in the cast iron pan.

Once all shells are done, pour remaining pasta sauce over the shells and top with slices of mozzarella cheese.

Bake for 30 minutes or until cheese is melty and brown bubbles start to appear.

Garnish with basil and serve hot.