Preheat your oven to 375°F and position rack in center. Lightly grease a 9x13-inch baking dish with nonstick spray.

Bring a large pot of salted water to a rolling boil. Add pasta shells and cook until al dente, about 8–10 minutes. Drain and set aside.

In a skillet over medium heat, sauté fresh spinach until wilted and bright green, about 2 minutes. Transfer spinach to a sieve, drain thoroughly and squeeze out all excess moisture.

In a mixing bowl, combine drained spinach, ricotta cheese, grated Parmesan, beaten egg, minced garlic, salt, and pepper. Stir until smooth and fully combined.

Using a small spoon or piping bag, fill each cooked shell with about 2 tablespoons of spinach-ricotta mixture.

Spread half of the marinara sauce evenly across the bottom of the greased baking dish. Arrange stuffed shells upright, then spoon remaining sauce over tops and sprinkle with mozzarella.

Cover the dish with foil and bake at 375°F for 20 minutes. Remove foil and bake an additional 10 minutes until cheese is golden and bubbling.