Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a boil and cook the jumbo pasta shells until al dente (about 8-10 minutes). Drain and rinse with cold water.

Heat olive oil in a large skillet over medium heat. Add diced onion and sauté for 3-4 minutes until translucent. Add minced garlic and sauté for another minute.

Stir in the fresh spinach and cook for 2-3 minutes until wilted. Remove from heat and cool slightly.

In a large mixing bowl, combine the cooked spinach mixture with ricotta, mozzarella, Parmesan, egg, pepper, and salt. Mix until well combined.

Spread a thin layer of marinara sauce on the bottom of a baking dish.

Stuff each pasta shell with the cheese-spinach filling and place them seam-side up in the dish.

Drizzle the remaining marinara sauce over the stuffed shells and sprinkle with extra Parmesan cheese.

Cover the dish with foil and bake for 25 minutes, remove the foil and bake for another 10 minutes until bubbly and golden.

Let cool for a few minutes, garnish with fresh basil and extra Parmesan before serving.