Preheat your oven to 425°F (220°C).

Steam the fresh spinach for about 1 minute until it wilts down.

In a large pot, bring salted water to a rolling boil, then add the large pasta shells. Cook for approximately 10 minutes, then drain.

In a mixing bowl, combine the chopped spinach, ricotta cheese, grated pecorino, minced garlic, dried oregano, lemon zest, red pepper flakes, salt, and black pepper.

Spread marinara sauce evenly across the bottom of a 9x13 inch baking dish.

Stuff each pasta shell with the creamy spinach and cheese mixture and place them upright in the baking dish.

Cover the baking dish tightly with aluminum foil and bake for 20 minutes.

Remove from the oven and let cool for a minute before serving.