Preheat oven to 191 °C (190 °C). Cook jumbo pasta shells according to package directions until al dente. Drain and rinse with cold water to prevent sticking. Set aside.

In a large bowl, combine ricotta cheese, softened cream cheese, grated Parmesan cheese, 120 ml of shredded mozzarella cheese, egg, finely chopped spinach, minced garlic, Italian seasoning, salt, and pepper. Mix thoroughly until well combined.

Spread about 240 ml of marinara sauce evenly over the bottom of a 9x13 inch baking dish.

Carefully spoon the cheese and spinach mixture into each cooked pasta shell, filling them generously. Arrange the filled shells in a single layer over the marinara sauce in the baking dish.

Pour the remaining marinara sauce over the stuffed shells, ensuring they are mostly covered. Drizzle with 15 ml of olive oil.

Cover the baking dish tightly with aluminum foil. Bake for 25 minutes.

Remove the foil, sprinkle the remaining 120 ml of mozzarella cheese evenly over the top. Return to the oven, uncovered, and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly and the sauce is heated through.

Let the stuffed shells rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.