Preheat the oven to 425°F.

Place the spinach in a steamer basket and set over a pot with 1-inch of water. Bring the water to a simmer, cover, and let steam for 1 minute, until the spinach is wilted. Transfer to a strainer and squeeze out the excess water and chop the spinach.

In a large pot of salted boiling water, cook the pasta shells for 10 minutes, until al dente. Drain and drizzle with a little olive oil to keep them from sticking together.

In a medium bowl, combine the spinach with the ricotta, pecorino, garlic, oregano, lemon zest, red pepper flakes, salt, and several grinds of pepper.

Spread the marinara in the bottom of a 9x13 baking dish. Stuff each shell with the filling and place in the dish. Cover with foil and bake for 20 minutes. Serve with more marinara on the side.