Start by steaming fresh spinach in a pot over medium heat for about 1 minute, just until wilted. Remove from heat, chop it finely, and set aside to cool slightly while you prep the pasta shells.

In a large pot, bring salted water to a boil and add the large pasta shells. Cook for approximately 10 minutes until they are al dente, stirring occasionally to prevent sticking. Once cooked, drain the shells thoroughly and toss them with a drizzle of olive oil.

In a mixing bowl, combine the finely chopped spinach, ricotta cheese, chopped pecorino cheese, minced garlic, dried oregano, lemon zest, red pepper flakes, salt, and black pepper. Stir until everything is well blended into a creamy filling.

Spread a generous layer of marinara sauce on the bottom of a 9x13 baking dish. Stuff each pasta shell with the prepared filling and place them in a single layer in the baking dish.

Preheat your oven to 425°F (220°C). Cover the baking dish with aluminum foil and bake for about 20 minutes, then remove the foil for the last 5 minutes to allow the tops to turn golden and bubbly. Serve hot, ideally with extra marinara sauce on the side.