Preheat the oven to 350°F (175°C) and lightly spray a 9×13 baking dish with cooking spray; set aside.
Bring a large pot of salted water to a boil and cook the jumbo shells according to package directions until al dente (about 10–12 minutes). Drain, rinse with cold water, toss with a little olive oil to prevent sticking, and set aside.
While the pasta cooks, heat 1 tablespoon olive oil in a large skillet over medium heat. Add the minced garlic and fresh spinach and sauté, stirring frequently, until the spinach is wilted and the garlic is fragrant, about 3–4 minutes.
Remove the skillet from heat and let the spinach cool about 5 minutes. Use paper towels to squeeze out excess moisture, then transfer to a cutting board and finely chop.
In a large bowl, combine the chopped spinach, ricotta, shredded Parmesan, egg, salt, pepper, dried parsley, dried basil, and dried oregano; stir until evenly mixed and taste to adjust seasoning if desired.
Spread about 3–4 tablespoons of marinara sauce over the bottom of the prepared baking dish to create a thin layer.
Fill each cooked jumbo shell with about 1½ tablespoons of the ricotta-spinach mixture and place the filled shells, seam side up, in the baking dish.
Pour roughly 2 to 2½ cups of the marinara sauce evenly over the filled shells (reserve a bit if desired); then sprinkle the shredded mozzarella evenly on top.
Bake on the center rack for about 30 minutes, or until the edges are bubbling and the cheese is lightly golden. Rotate the pan halfway through baking for even browning.
Remove from the oven and let rest a few minutes, then garnish with chopped fresh parsley if desired and serve.
