1) Preheat the oven to 425°F.
2) 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.
3) 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.
4) In a medium bowl, combine the spinach with the ricotta, pecorino, garlic, oregano, lemon zest, red pepper flakes, salt, and several grinds of pepper.
5) Spread the marinara in the bottom of a 9x13 baking dish.
6) Stuff each shell with the filling and place in the dish. Cover with foil and bake for 20 minutes.
7) Serve with more marinara on the side.