Heat the skillet to medium heat. Add the ground sausage to the skillet. Break it apart with a wooden spoon to create small pieces as the meat cooks. Once the sausage is mostly cooked, stir in the chopped onions. Continue to stir and sauté for 2-3 minutes. Add the marinara and water. Then simmer for 5-10 minutes and turn off the heat. Pour the meat sauce into the baking dish.

Meanwhile, set a pot of water over high heat and bring to a boil. Throw a large pinch of salt into the water. Once boiling, add the pasta shells and cook for 5-10 minutes, according to the package instructions. You want the shells to be cooked, so they are soft and flexible, but not overly cooked where they will rip when you fill them. The moment the shells are perfectly cooked, use a slotted spoon to remove them from the boiling water and set them on a clean work surface.

Set out a mixing bowl. Open the package of chopped spinach and squeeze it thoroughly to remove as much moisture as possible. Use paper towels if needed to dry out the spinach. Combine the ricotta cheese, Parmesan, egg, minced garlic, dried chopped spinach, Italian seasoning, 1 teaspoon salt, and ½ teaspoon black pepper in the bowl. Mix well.

Use a spoon to stuff the cooked pasta shells with the cheese filling. It's OK if they are slightly overstuffed and the filling is exposed.

Place each stuffed shell into the baking dish, pressing them down into the meat sauce.

Once all the shells are stuffed and in the baking dish, sprinkle the top with mozzarella cheese.

Cover the baking dish with foil, and bake for 30 minutes. *You can spray the foil with no nonstick cooking spray, if you like, to keep the cheese from sticking to the foil.