Preheat oven to 375 degrees F. Cook pasta shells according to package directions. Drain and rinse under cold water to stop cooking.

In a medium skillet, cook sausage over medium heat, about 10 minutes. Break it apart as it cooks. Drain excess oil from sausage in a colander. Combine cooked sausage, half of the mozzarella, parmesan, ricotta, salt and pepper in a large bowl. Using a paper towel, squeeze out the water from the thawed spinach. Add to the bowl with remaining ingredients and mix until well combined.

Spread half of marinara onto bottom of large baking dish.

Using a spoon, generously fill each shell with the sausage/cheese mixture and assemble into the baking dish. Spoon the remaining marinara sauce over the shells and sprinkle with remaining mozzarella on top. Bake for 20 minutes, until cheese is melted.