Preheat the oven to 375 degrees and grease a 13x9-inch baking dish with some oil.
Spread 1/3 cup marinara sauce evenly into the dish and set aside.
Bring a large pot of water to a boil. Add a pinch of salt and pasta. Stir well and cook for 10 minutes or a little less than al dente. Once done, drain and rinse with the cold water.
Next, boil a kettle full of water. Place the spinach in a colander and set it over the sink.
Now carefully pour the boiling water over the spinach so the hot water wilts the leaves.
Once the leaves are wilted, immediately run under the cold water. Squeeze as much water as you can from the spinach leaves, then chop into bite-size pieces.
In a large mixing bowl, combine ricotta cheese, parmesan cheese, and half of the mozzarella cheese. Stir in chopped spinach and crack an egg. Add garlic, basil, salt, and black pepper; mix until very well combined.
Spoon the cheese mixture into the pasta shells and place it into the prepared baking dish, nestling them in the sauce.
Pour remaining marinara sauce over shells and drizzle with the olive oil. Sprinkle the remaining mozzarella cheese on top.
Cover the dish with aluminum foil and bake for 30 minutes until cheese on top is melted and the sauce is bubbly. Remove from the oven and serve warm.