Preheat your oven to 375°F (190°C).
Cook the pasta shells according to the package instructions until al dente. Drain and set aside.
In a large skillet, heat a drizzle of oil over medium heat and sauté minced garlic for about 30 seconds until fragrant.
Add chopped spinach to the skillet and cook until wilted, about 3-4 minutes. Remove from heat and let cool slightly.
In a bowl, combine ricotta, mozzarella, Parmesan, the cooked spinach, beaten egg, salt, and pepper. Mix until well combined.
Stuff each cooked pasta shell with the filling mixture and place them in a baking dish seam side up.
Pour marinara sauce over the stuffed shells, ensuring they are well covered.
Sprinkle additional mozzarella cheese on top for an extra cheesy layer.
Cover with aluminum foil and bake for 25 minutes, then remove foil and bake for an additional 10-15 minutes until bubbly and golden.
Let the dish sit for 5-10 minutes before serving. Garnish as desired.