Preheat your oven to 425°F (220°C). Gather your equipment including a large pot, a steamer basket, and a 9x13 baking dish.

Steam the spinach in the steamer basket over simmering water for about 1 minute until wilted.

Drain excess liquid from the spinach by squeezing it, then chop finely.

Cook the jumbo pasta shells in salted boiling water for about 10 minutes until al dente. Drain and drizzle with olive oil.

In a mixing bowl, combine chopped spinach, ricotta, pecorino, garlic, oregano, lemon zest, red pepper flakes, salt, and pepper. Mix until smooth.

Spread a layer of marinara sauce in the bottom of the baking dish. Stuff each shell with the filling and place in the dish.

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

Remove the foil and serve with extra marinara sauce if desired.