Preheat the oven to 425°F (220°C).

In a medium saucepan, bring water to a boil and steam the spinach for 1 minute. Drain and chop finely.

In a large pot of salted water, cook the pasta shells for about 10 minutes until al dente. Drain and drizzle with olive oil.

In a medium bowl, combine spinach, ricotta, pecorino, garlic, oregano, lemon zest, red pepper flakes, salt, and pepper until well mixed.

Spread marinara sauce in the bottom of a 9x13 baking dish.

Stuff each pasta shell with the filling and place them seam-side up in the dish.

Cover with aluminum foil and bake for 20 minutes.

Remove from the oven and let it cool for a few minutes before serving.