Bring a large pot of salted water to a boil. Add the jumbo shells. Cook until just al dente. Do not overcook them. They will cook more in the oven. Drain the shells and rinse gently with cool water. Set them aside carefully.

Heat olive oil in a skillet over medium heat. Add minced garlic. Cook for one minute. Add chopped spinach. Cook until soft. This takes about two minutes. Remove from heat and let it cool slightly before mixing.

In a large bowl, add ricotta, mozzarella and Parmesan. Crack in the egg. Add oregano, salt, and black pepper. Stir well until smooth. Add the cooked spinach mixture. Mix gently until everything is well combined and creamy.

Spread one cup of marinara sauce on the bottom of the baking dish. This keeps the shells moist. It also adds flavor to every bite. Preheat your oven to 375°F while you fill the shells.

Take one shell at a time. Use a spoon to fill it with the cheese mixture. Do not overfill. Place each stuffed shell in the baking dish. Arrange them in a single layer over the sauce.

Pour the remaining marinara sauce over the stuffed shells. Make sure they are covered lightly. Sprinkle extra mozzarella on top. This will melt and turn golden in the oven.

Cover the dish with foil. Bake for 25 minutes. Remove the foil and bake another 10 minutes. The cheese should bubble and turn slightly golden. Let it rest for 5 minutes before serving.